Fix version comparisons for Python 3.6, 3.10 and 4 (#32389)
Summary:
There's some code which uses `six.PY3`, similar to:
```python
if six.PY3:
print("Python 3+ code")
else:
print "Python 2 code"
```
Where:
```python
PY3 = sys.version_info[0] == 3
```
When run on Python 4, this will run the Python 2 code! Instead, use `six.PY2` and avoid `six.PY3`.
---
Similarly, there's some `sys.version_info[0] == 3` checks, better done as `sys.version_info[0] >= 3`.
---
Also, it's better to avoid comparing the `sys.version` string, as it makes assumptions that each version component is exactly one character long, which will break in Python 3.10:
```pycon
>>> sys.version
'3.8.1 (v3.8.1:1b293b6006, Dec 18 2019, 14:08:53) \n[Clang 6.0 (clang-600.0.57)]'
>>> sys.version < "3.3"
False
>>> fake_v3_10 = '3.10.1 (v3.8.1:1b293b6006, Dec 18 2019, 14:08:53) \n[Clang 6.0 (clang-600.0.57)]'
>>> fake_v3_10 < "3.3"
True
```
---
Finally, I think the intention here is to skip when the Python version is < 3.6:
```python
unittest.skipIf(sys.version_info[0] < 3 and sys.version_info[1] < 6, "dict not ordered")
```
However, it will really skip for Python 0.0-0.5, 1.0-1.5 and 2.0-2.5. It's best to compare to the `sys.version_info` tuple and not `sys.version_info[1]`:
```python
unittest.skipIf(sys.version_info < (3, 6), "dict not ordered")
```
---
Found using https://github.com/asottile/flake8-2020:
```console
$ pip install -U flake8-2020
$ flake8 --select YTT
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32389
Reviewed By: zou3519
Differential Revision: D24424662
Pulled By: ezyang
fbshipit-source-id: 1266c4dbcc8ae4d2e2e9b1d7357cba854562177c
