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.

まず、sys.version_info と sys.platform に関係する条件に対する mypy の既存の静的評価を見つけます。バージョン比較がどのように表現され、テストされているかを追跡し、次に sys.hexversion と整数リテラルの比較で期待される動作を判断します。既存のケースと一貫した形でサポート対象の hexversion 条件が評価され、要求された比較をカバーするテストがあれば完了です。