CLN: Remove PY35 flag from pandas.compat
- [X] xref #25725
- [X] tests passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The `PY35` flag is always `True` after dropping Python 2, as Python 3.5 is now the lowest supported version.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26036?src=pr&el=h1) Report
> Merging [#26036](https://codecov.io/gh/pandas-dev/pandas/pull/26036?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9d66bdf1c6f252ba8a9f7b3298877d9c021a84c9?src=pr&el=desc) will **increase** coverage by `0.05%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26036/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26036?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26036      +/-   ##
==========================================
+ Coverage   91.84%   91.89%   +0.05%     
==========================================
  Files         175      175              
  Lines       52517    52478      -39     
==========================================
- Hits        48232    48224       -8     
+ Misses       4285     4254      -31
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <100%> (+0.05%)` | :arrow_up: |
| #single | `40.74% <100%> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26036?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/26036/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `77.64% <ø> (-0.26%)` | :arrow_down: |
| [pandas/core/computation/expr.py](https://codecov.io/gh/pandas-dev/pandas/pull/26036/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXhwci5weQ==) | `96.7% <100%> (+8.14%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26036/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26036/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26036/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.72% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26036?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/26036?src=pr&el=footer). Last update [9d66bdf...30619bc](https://codecov.io/gh/pandas-dev/pandas/pull/26036?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/26036?src=pr&el=h1) Report
> Merging [#26036](https://codecov.io/gh/pandas-dev/pandas/pull/26036?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9d66bdf1c6f252ba8a9f7b3298877d9c021a84c9?src=pr&el=desc) will **increase** coverage by `0.05%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26036/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26036?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26036      +/-   ##
==========================================
+ Coverage   91.84%   91.89%   +0.05%     
==========================================
  Files         175      175              
  Lines       52517    52478      -39     
==========================================
- Hits        48232    48224       -8     
+ Misses       4285     4254      -31
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <100%> (+0.05%)` | :arrow_up: |
| #single | `40.74% <100%> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26036?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/26036/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `77.64% <ø> (-0.26%)` | :arrow_down: |
| [pandas/core/computation/expr.py](https://codecov.io/gh/pandas-dev/pandas/pull/26036/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXhwci5weQ==) | `96.7% <100%> (+8.14%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26036/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26036/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26036/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.72% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26036?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/26036?src=pr&el=footer). Last update [9d66bdf...30619bc](https://codecov.io/gh/pandas-dev/pandas/pull/26036?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @jschendel 