CLN/DEPS: Clean up post numpy bump to 1.12
- [x] follow-up to #23062 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I stumbled over a place I had forgotten to change in #23062 - `README.md`. And since I recently found some `grep`-like powers on windows with `findstr`, I took this opportunity to have a look through the code base for other things that got missed in that PR.

Turns out there was quite a bit of crusty old code left from before the *last* bump to `numpy 1.9` (the oldest I found was some commented out code due to segfaults for `numpy < 1.5.1`).

Also, since #23062 bumped matplotlib to `2.0.0`, I looked for old code there as well.

There were a few places where it was not 100% clear what the right way to remove them was - I'm relying on a passing test suite to be a good enough yardstick. In any case, this should have a thorough review to make sure I didn't eff something up.

I'll add a more detailed review right away to explain my reasoning behind each one of the changes.
looks fine. needs a rebase and everything passing. ping when green. if something is not working after trying, can just mark it and come back with a followup. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23796?src=pr&el=h1) Report
> Merging [#23796](https://codecov.io/gh/pandas-dev/pandas/pull/23796?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a7b187ab42c48342478ed8664145f21578f50622?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23796/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23796?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23796      +/-   ##
==========================================
+ Coverage   92.29%   92.31%   +0.01%     
==========================================
  Files         161      161              
  Lines       51498    51464      -34     
==========================================
- Hits        47530    47507      -23     
+ Misses       3968     3957      -11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <100%> (+0.01%)` | :arrow_up: |
| #single | `42.42% <90.9%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23796?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/packers.py](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhY2tlcnMucHk=) | `88.08% <ø> (ø)` | :arrow_up: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.34% <ø> (-0.02%)` | :arrow_down: |
| [pandas/plotting/\_misc.py](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19taXNjLnB5) | `38.68% <ø> (-0.31%)` | :arrow_down: |
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `95.96% <ø> (ø)` | :arrow_up: |
| [pandas/core/arrays/sparse.py](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3NwYXJzZS5weQ==) | `91.93% <ø> (-0.02%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.48% <ø> (+0.28%)` | :arrow_up: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.59% <100%> (-0.03%)` | :arrow_down: |
| [pandas/core/reshape/tile.py](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS90aWxlLnB5) | `94.73% <100%> (ø)` | :arrow_up: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `89.62% <100%> (+0.63%)` | :arrow_up: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `93.71% <100%> (+0.03%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/pandas-dev/pandas/pull/23796/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23796?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23796?src=pr&el=footer). Last update [a7b187a...ce5bd94](https://codecov.io/gh/pandas-dev/pandas/pull/23796?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @h-vetinari! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/categorical.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/core/arrays/categorical.py) !

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimelike.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/core/arrays/datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/core/arrays/sparse.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/core/arrays/sparse.py) !

 - There are no PEP8 issues in the file [`pandas/core/dtypes/cast.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/core/dtypes/cast.py) !

 - There are no PEP8 issues in the file [`pandas/core/dtypes/dtypes.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/core/dtypes/dtypes.py) !

 - There are no PEP8 issues in the file [`pandas/core/dtypes/missing.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/core/dtypes/missing.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimes.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/core/indexes/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/core/internals/blocks.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/core/internals/blocks.py) !

 - There are no PEP8 issues in the file [`pandas/core/internals/managers.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/core/internals/managers.py) !

 - There are no PEP8 issues in the file [`pandas/core/reshape/tile.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/core/reshape/tile.py) !

 - There are no PEP8 issues in the file [`pandas/io/packers.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/io/packers.py) !

 - There are no PEP8 issues in the file [`pandas/io/pickle.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/io/pickle.py) !

 - There are no PEP8 issues in the file [`pandas/plotting/_misc.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/plotting/_misc.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arrays/categorical/test_repr.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/arrays/categorical/test_repr.py) !

 - There are no PEP8 issues in the file [`pandas/tests/dtypes/test_inference.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/dtypes/test_inference.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_constructors.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/frame/test_constructors.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_operators.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/frame/test_operators.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/common.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/indexes/common.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/multi/test_analytics.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/indexes/multi/test_analytics.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/test_chaining_and_caching.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/indexing/test_chaining_and_caching.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/test_iloc.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/indexing/test_iloc.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/test_pytables.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/io/test_pytables.py) !

 - There are no PEP8 issues in the file [`pandas/tests/plotting/test_datetimelike.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/plotting/test_datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/tests/plotting/test_frame.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/plotting/test_frame.py) !

 - There are no PEP8 issues in the file [`pandas/tests/plotting/test_series.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/plotting/test_series.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_analytics.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/series/test_analytics.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_base.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/test_base.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_nanops.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/test_nanops.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_panel.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/test_panel.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_sorting.py`](https://github.com/h-vetinari/pandas/blob/ecfe8248a856594aaa3d10c758e1b724ba2e2024/pandas/tests/test_sorting.py) !

 thanks @h-vetinari nice cleanup