CLN/INT: Rename _possibly to _maybe (GH15764)
Also rename "private" functions in pandas.type.cast

 - [x] closes #15764
 - [x] tests passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
thanks @StanczakDominik exactly what I was looking.

ping on green. (hopefully linting passes too!). couple of linting errors.


```
pandas/computation/expr.py:386:56: E127 continuation line over-indented for visual indent
pandas/core/internals.py:5144:80: E501 line too long (88 > 79 characters)
``` # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15771?src=pr&el=h1) Report
> Merging [#15771](https://codecov.io/gh/pandas-dev/pandas/pull/15771?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/163d18ed0d46eeb375f8170f1044808ff40b2a65?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `97.01%`.


```diff
@@            Coverage Diff             @@
##           master   #15771      +/-   ##
==========================================
- Coverage   91.01%      91%   -0.02%     
==========================================
  Files         143      143              
  Lines       49377    49403      +26     
==========================================
+ Hits        44941    44958      +17     
- Misses       4436     4445       +9
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15771?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tools/util.py](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL3V0aWwucHk=) | `96.84% <100%> (ø)` | :arrow_up: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.51% <100%> (ø)` | :arrow_up: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `96.89% <100%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.86% <100%> (-0.1%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.24% <100%> (ø)` | :arrow_up: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `94.96% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape.py](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS5weQ==) | `99.27% <100%> (ø)` | :arrow_up: |
| [pandas/tseries/tdi.py](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvdGRpLnB5) | `90.23% <100%> (ø)` | :arrow_up: |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.78% <100%> (ø)` | :arrow_up: |
| [pandas/sparse/array.py](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?src=pr&el=tree#diff-cGFuZGFzL3NwYXJzZS9hcnJheS5weQ==) | `91.46% <100%> (ø)` | :arrow_up: |
| ... and [20 more](https://codecov.io/gh/pandas-dev/pandas/pull/15771?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15771?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/15771?src=pr&el=footer). Last update [163d18e...486b932](https://codecov.io/gh/pandas-dev/pandas/compare/163d18ed0d46eeb375f8170f1044808ff40b2a65...486b932100fd17418982b25cda4039818007b6a5?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). Well, I guess I must have missed them somehow.

I'm not entirely sure why Codecov claims this would decrease coverage. It might be related to my comment above.

By the way, as this is my first foray into giving back to the awesome packages in the Python ecosystem, I'm not entirely sure if my thinking is correct: once test pass, I should squish my commits into one for inclusion in master, right? no worries @StanczakDominik for future reference: http://pandas-docs.github.io/pandas-docs-travis/contributing.html#python-pep8 thanks @StanczakDominik 