Make signal global enum constants annotated as literals

[bswck]

Apart from regular rewrites of annotations from GLOBAL: EnumClassName to GLOBAL: Literal[EnumClassName.MEMBER], you may notice a small different kind of change:

-    SIG_BLOCK = Sigmasks.SIG_BLOCK
-    SIG_UNBLOCK = Sigmasks.SIG_UNBLOCK
-    SIG_SETMASK = Sigmasks.SIG_SETMASK
+    SIG_BLOCK: Literal[Sigmasks.SIG_BLOCK]
+    SIG_UNBLOCK: Literal[Sigmasks.SIG_UNBLOCK]
+    SIG_SETMASK: Literal[Sigmasks.SIG_SETMASK]

The reason why I did is because this unifies the outcomes across pyright and mypy (reveal_type(SIG_BLOCK) for old SIG_BLOCK is Sigmasks according to mypy and Literal[Sigmasks.SIG_BLOCK] according to pyright) -- with the new version, both checkers see literal types.

[bswck]

Interesting failures. I can see the concern mainly being that globals prefer non-alias enum members, but the literal integer values at runtime are effectively correct.

[bswck]

Added # alias comments for clarity and intention assurance.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[sobolevn]

We suspect that this is the cause of python/mypy#18628