manifest: delay import of typing.ByteString for py 3.14 support (is…

…sue6940)

Since Python 2.7 and 3.5, `typing.ByteString` was defined as an alias for
`bytes | bytearray | memoryview`, and `bytes` was also accepted as a shorthand
for this, so we have `bytes` sprinkled all over the codebase.  But then PEP-688
reversed all of that by deprecating `typing.ByteString` and its successor
`collections.abc.ByteString` in Python 3.12 (as well as the `bytes` shorthand)[1],
and removing it completely in Python 3.14.  That leaves us with a couple of
problems, namely defining something useful that spans py3.8-py3.13 and keeps
pytype happy, and finding all of the instances where `bytes` doesn't really mean
`bytes`.  The current successor to all of this is `collections.abc.Buffer` in
Python 3.12 (or `typing_extensions.Buffer` in previous versions).  However, the
current CI does type checking using Python 3.11 (so the former is not avaiable),
and pytype has issues with importing `typing_extensions.Buffer`[2].

The good news is we don't need to deal with this mess immediately, since the
type annotation evaluation is delayed to the type checking phase, and we're
making no effort at supporting it in all supported versions of Python.  So by
delaying the import of this particular symbol, we can still use it for type
checking purposes, but can start assessing Python 3.14 problems without doing a
lot of extra work.  Putting this on stable will allow people interested in 3.14
to work on it 4-5 extra months earlier (and apparently there's some interest).

[1] https://peps.python.org/pep-0688/#no-special-meaning-for-bytes
[2] google/pytype#1772

mercurial/manifest.py

@@ -10,10 +10,10 @@
 import heapq
 import itertools
 import struct
+import typing
 import weakref
 
 from typing import (
-    ByteString,
     Callable,
     Collection,
     Dict,
@@ -51,6 +51,12 @@
     constants as revlog_constants,
 )
 
+if typing.TYPE_CHECKING:
+    from typing import (
+        ByteString,
+    )
+
+
 parsers = policy.importmod('parsers')
 propertycache = util.propertycache