typing: add type hints to `mercurial.policy`

typing: add type hints to `mercurial.policy`

Mostly trivial, but this seems like the logical module to use to inject the
hints from `cext`, `pure`, etc, given that this file has the fallback policy.
This is a first step.

There doesn't appear to be a predefined type for a module in py3.7, so those are
omitted for now.

--- a/mercurial/policy.py	Sat Jul 20 01:55:09 2024 -0400
+++ b/mercurial/policy.py	Sat Jul 20 17:03:30 2024 -0400
@@ -8,6 +8,14 @@
 
 import os
 import sys
+import typing
+
+if typing.TYPE_CHECKING:
+    from typing import (
+        Dict,
+        Optional,
+        Tuple,
+    )
 
 # Rules for how modules can be loaded. Values are:
 #
@@ -23,8 +31,8 @@
 # By default, fall back to the pure modules so the in-place build can
 # run without recompiling the C extensions. This will be overridden by
 # __modulepolicy__ generated by setup.py.
-policy = b'allow'
-_packageprefs = {
+policy: bytes = b'allow'
+_packageprefs: "Dict[bytes, Tuple[Optional[str], Optional[str]]]" = {
     # policy: (versioned package, pure package)
     b'c': ('cext', None),
     b'allow': ('cext', 'pure'),
@@ -39,7 +47,7 @@
 try:
     from . import __modulepolicy__  # type: ignore
 
-    policy = __modulepolicy__.modulepolicy
+    policy: bytes = __modulepolicy__.modulepolicy
 except ImportError:
     pass
 
@@ -48,14 +56,14 @@
 # The canonical way to do this is to test platform.python_implementation().
 # But we don't import platform and don't bloat for it here.
 if '__pypy__' in sys.builtin_module_names:
-    policy = b'cffi'
+    policy: bytes = b'cffi'
 
 # Environment variable can always force settings.
 if 'HGMODULEPOLICY' in os.environ:
-    policy = os.environ['HGMODULEPOLICY'].encode('utf-8')
+    policy: bytes = os.environ['HGMODULEPOLICY'].encode('utf-8')
 
 
-def _importfrom(pkgname, modname):
+def _importfrom(pkgname: str, modname: str):
     # from .<pkgname> import <modname> (where . is looked through this module)
     fakelocals = {}
     pkg = __import__(pkgname, globals(), fakelocals, [modname], level=1)
@@ -69,7 +77,7 @@
 
 
 # keep in sync with "version" in C modules
-_cextversions = {
+_cextversions: "Dict[Tuple[str, str], int]" = {
     ('cext', 'base85'): 1,
     ('cext', 'bdiff'): 3,
     ('cext', 'mpatch'): 1,
@@ -78,7 +86,7 @@
 }
 
 # map import request to other package or module
-_modredirects = {
+_modredirects: "Dict[Tuple[str, str], Tuple[str, str]]" = {
     ('cext', 'charencode'): ('cext', 'parsers'),
     ('cffi', 'base85'): ('pure', 'base85'),
     ('cffi', 'charencode'): ('pure', 'charencode'),
@@ -86,7 +94,7 @@
 }
 
 
-def _checkmod(pkgname, modname, mod):
+def _checkmod(pkgname: str, modname: str, mod) -> None:
     expected = _cextversions.get((pkgname, modname))
     actual = getattr(mod, 'version', None)
     if actual != expected:
@@ -97,7 +105,7 @@
         )
 
 
-def importmod(modname):
+def importmod(modname: str):
     """Import module according to policy and check API version"""
     try:
         verpkg, purepkg = _packageprefs[policy]
@@ -118,12 +126,12 @@
     return _importfrom(pn, mn)
 
 
-def _isrustpermissive():
+def _isrustpermissive() -> bool:
     """Assuming the policy is a Rust one, tell if it's permissive."""
     return policy.endswith(b'-allow')
 
 
-def importrust(modname, member=None, default=None):
+def importrust(modname: str, member: "Optional[str]" = None, default=None):
     """Import Rust module according to policy and availability.
 
     If policy isn't a Rust one, this returns `default`.