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.

Comienza localizando la evaluación estática existente de mypy para las condiciones que implican sys.version_info y sys.platform. Sigue cómo se representan y prueban las comparaciones de versiones y, a continuación, determina el comportamiento esperado para las comparaciones entre sys.hexversion y literales enteros. Se considerará completado cuando las condiciones de hexversion compatibles se evalúen de forma coherente con los casos existentes y haya pruebas que cubran las comparaciones solicitadas.