CLN: Remove PY2/3 references in pandas/util
xref #25725.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25962?src=pr&el=h1) Report
> Merging [#25962](https://codecov.io/gh/pandas-dev/pandas/pull/25962?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f90f4aad4589727ebb3235326970a9110aaa309f?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25962/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25962?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25962      +/-   ##
==========================================
- Coverage   91.84%   91.83%   -0.01%     
==========================================
  Files         175      175              
  Lines       52553    52550       -3     
==========================================
- Hits        48269    48261       -8     
- Misses       4284     4289       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.39% <100%> (-0.01%)` | :arrow_down: |
| #single | `41.9% <100%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25962?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/\_validators.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3ZhbGlkYXRvcnMucHk=) | `96.8% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.61% <100%> (-0.13%)` | :arrow_down: |
| [pandas/util/\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RlY29yYXRvcnMucHk=) | `91.42% <100%> (-0.09%)` | :arrow_down: |
| [pandas/util/\_test\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3Rlc3RfZGVjb3JhdG9ycy5weQ==) | `93.22% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/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/25962/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25962?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/25962?src=pr&el=footer). Last update [f90f4aa...b899871](https://codecov.io/gh/pandas-dev/pandas/pull/25962?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/25962?src=pr&el=h1) Report
> Merging [#25962](https://codecov.io/gh/pandas-dev/pandas/pull/25962?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f90f4aad4589727ebb3235326970a9110aaa309f?src=pr&el=desc) will **decrease** coverage by `1.45%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25962/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25962?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25962      +/-   ##
==========================================
- Coverage   91.84%   90.39%   -1.46%     
==========================================
  Files         175      175              
  Lines       52553    52550       -3     
==========================================
- Hits        48269    47503     -766     
- Misses       4284     5047     +763
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.39% <100%> (-0.01%)` | :arrow_down: |
| #single | `?` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25962?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/\_validators.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3ZhbGlkYXRvcnMucHk=) | `96.8% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.61% <100%> (-0.13%)` | :arrow_down: |
| [pandas/util/\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RlY29yYXRvcnMucHk=) | `91.42% <100%> (-0.09%)` | :arrow_down: |
| [pandas/util/\_test\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3Rlc3RfZGVjb3JhdG9ycy5weQ==) | `93.22% <100%> (ø)` | :arrow_up: |
| [pandas/io/feather\_format.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2ZlYXRoZXJfZm9ybWF0LnB5) | `21.05% <0%> (-68.43%)` | :arrow_down: |
| [pandas/core/computation/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vcHl0YWJsZXMucHk=) | `62.5% <0%> (-27.75%)` | :arrow_down: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `63.43% <0%> (-26.8%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/computation/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vY29tbW9uLnB5) | `84.21% <0%> (-5.27%)` | :arrow_down: |
| [pandas/core/computation/expr.py](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXhwci5weQ==) | `84.78% <0%> (-3.75%)` | :arrow_down: |
| ... and [6 more](https://codecov.io/gh/pandas-dev/pandas/pull/25962/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25962?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/25962?src=pr&el=footer). Last update [f90f4aa...c945910](https://codecov.io/gh/pandas-dev/pandas/pull/25962?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. merge on green.