Step 1 of code clean-up: mechanically remove if-else branches that would never
run in Python/PyPy >= 3.7.  There's still some unused or redundant code, left
for the next step to facilitate review.

Note: all the remaining `sys.hexversion` tests were standardized to use
numerical comparison and hexadecimal integer literals (e.g. `0x30b00a7` for
version 3.11.0a7) as most of them already were.

…ic where it's obvious