typing: add a type hint to `mercurial/hg.py`

Somewhere between hg 3dbc7b1ecaba and hg 8e3f6b5bf720, the first value of the
tuple changed from bytes to str.  Let's lock this in, so that pytype flags it
if someone mistakenly adds a tuple with bytes somewhere.

mercurial/hg.py

@@ -11,6 +11,7 b' import os'
 import posixpath
 import shutil
 import stat
+import typing
 import weakref
 
 from .i18n import _
@@ -57,6 +58,11 b' from .utils import ('
     urlutil,
 )
 
+if typing.TYPE_CHECKING:
+    from typing import (
+        List,
+        Tuple,
+    )
 
 release = lock.release
 
@@ -1597,7 +1603,7 b' def remoteui(src, opts):'
 # Files of interest
 # Used to check if the repository has changed looking at mtime and size of
 # these files.
-foi = [
+foi: "List[Tuple[str, bytes]]" = [
     ('spath', b'00changelog.i'),
     ('spath', b'phaseroots'),  # ! phase can change content at the same size
     ('spath', b'obsstore'),