Conditional deduction support with comparison of `sys.hexversion` against integer literal

Feature

Currently, mypy statically evaluates some if branches involving only sys.version_info and sys.platform as always false based on the version and platform it is running on. I propose to have it recognize comparisons against sys.hexversion as well.

Pitch

Some specialized functions targeting bugs patched in a certain alpha, beta or release candidate or internal behavioural differences between patch versions would need this functionality. Also, it may be desirable to give alpha/beta/rc versions a grace period for certain small deprecations, only raising an error on their use in the final python release. Implementation-wise, because the exact value of hexversion can be easily retrieved, and sys.platform and sys.version_info have set precedents, I do not believe it would be unreasonably hard to implement.

Start by locating mypy’s existing static evaluation for conditions involving sys.version_info and sys.platform. Trace how version comparisons are represented and tested, then determine the expected behavior for comparisons between sys.hexversion and integer literals. Done means supported hexversion conditions are evaluated consistently with the existing cases, with tests covering the requested comparisons.