parser: force a `ValueError` to bytes before passing to `error.ParseError`

I'm not sure what changed before pytype 09-09-2021 (from 04-15-2021), but this
started getting flagged.  I think there's a pytype bug here, because I don't
see how `.lower()` can be getting called on a `ValueError` after it is forced to
a byte string.  That's suppressed for now to make progress.

This fixes:

    File "/mnt/c/Users/Matt/hg/mercurial/parser.py", line 219, in unescapestr: Function bytestr.__init__ was called with the wrong arguments [wrong-arg-types]
             Expected: (self, ints: Iterable[int])
      Actually passed: (self, ints: ValueError)
      Attributes of protocol Iterable[int] are not implemented on ValueError: __iter__
    File "/mnt/c/Users/Matt/hg/mercurial/parser.py", line 219, in unescapestr: No attribute 'lower' on ValueError [attribute-error]
      In Union[ValueError, mercurial.pycompat.bytestr]

Differential Revision: https://phab.mercurial-scm.org/D11471

mercurial/parser.py

@@ -21,7 +21,6 b' from __future__ import absolute_import, '
 from .i18n import _
 from . import (
     error,
-    pycompat,
     util,
 )
 from .utils import stringutil
@@ -216,7 +215,11 b' def unescapestr(s):'
         return stringutil.unescapestr(s)
     except ValueError as e:
         # mangle Python's exception into our format
-        raise error.ParseError(pycompat.bytestr(e).lower())
+        # TODO: remove this suppression.  For some reason, pytype 2021.09.09
+        #   thinks .lower() is being called on Union[ValueError, bytes].
+        # pytype: disable=attribute-error
+        raise error.ParseError(stringutil.forcebytestr(e).lower())
+        # pytype: enable=attribute-error
 
 
 def _prettyformat(tree, leafnodes, level, lines):