DEPS: Drop Python 3.4 support
Title is self-explanatory.

Closes #15251.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=h1) Report
> Merging [#16303](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ce4eef3750052cec62ca0fe6536521dec523cd64?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16303/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16303      +/-   ##
==========================================
- Coverage   90.39%   90.37%   -0.02%     
==========================================
  Files         161      161              
  Lines       50863    50863              
==========================================
- Hits        45977    45968       -9     
- Misses       4886     4895       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.16% <ø> (ø)` | :arrow_up: |
| #single | `40.33% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `61.77% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.59% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16303?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/16303?src=pr&el=footer). Last update [ce4eef3...a186ba8](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=h1) Report
> Merging [#16303](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0ea0f25bfb920cd0f58872dc95913a7aa9c0c3da?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16303/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16303      +/-   ##
==========================================
- Coverage   90.38%   90.36%   -0.03%     
==========================================
  Files         161      161              
  Lines       50916    50916              
==========================================
- Hits        46021    46009      -12     
- Misses       4895     4907      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.13% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.22% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `61.77% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.56% <0%> (-0.2%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.68% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16303?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/16303?src=pr&el=footer). Last update [0ea0f25...9c33e44](https://codecov.io/gh/pandas-dev/pandas/pull/16303?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 so we have a couple of circle ci jobs which test 3.4 move these to 3.6 i think. (they still hit a case which we need to test). @jreback : Good catch.  Copied and pasted the `requirements-3.6*` files to `requirements-3.6_SLOW*` files and also removed 3.4 from the `tox.ini` file (git diff is not very helpful, hence why I'm explaining my changes here). @jreback : Weird locale failure on Circle <a href="https://circleci.com/gh/pandas-dev/pandas/1875#tests/containers/2">here</a>.  Not sure what is wrong with the test files. @jreback : Everything is green here, but I presume you are waiting to merge this until AFTER `0.20.2` is released into the wild (correct me if I'm wrong). @gfyoung no, we can merge things for 0.21.0 now. @jorisvandenbossche had an objection. I actually have no problem with this. yes its not *hard* to support. But 3.4 is not very well supported by conda anymore, no point in us holding out the torch. @jreback : So is this good to go then? @jreback : Made the requested changes, and everything is still green.  PTAL. @jreback : Made the requested changes, and everything is still green.  PTAL. @jreback : Made the requested changes, and everything is still green. PTAL. thanks!