[Attn: mercurial maintainer] hg fails with python3.9

[Ken Brown]

/usr/bin/hg specifies /usr/bin/python3 in its shebang, but further down it has
  libdir = '../lib/python3.8/site-packages'

This causes hg to fail as follows if /usr/bin/python3 points to python3.9:

$ hg
Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/mercurial/policy.py", line 69, in 
_importfrom
    fakelocals[modname] = mod = getattr(pkg, modname)
  File "/usr/lib/python3.8/site-packages/mercurial/pycompat.py", line 317, in w
    return f(object, sysstr(name), *args)
AttributeError: module 'mercurial.cext' has no attribute 'parsers'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/hg", line 61, in <module>
    dispatch.run()
  File "/usr/lib/python3.9/importlib/util.py", line 245, in __getattribute__
    self.__spec__.loader.exec_module(self)
  File "<frozen importlib._bootstrap_external>", line 850, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/usr/lib/python3.8/site-packages/mercurial/dispatch.py", line 21, in 
<module>
    from .i18n import _
  File "/usr/lib/python3.9/importlib/util.py", line 245, in __getattribute__
    self.__spec__.loader.exec_module(self)
  File "/usr/lib/python3.8/site-packages/mercurial/i18n.py", line 123, in 
<module>
    if _plain():
  File "/usr/lib/python3.8/site-packages/mercurial/i18n.py", line 115, in _plain
    b'HGPLAIN' not in encoding.environ
  File "/usr/lib/python3.9/importlib/util.py", line 245, in __getattribute__
    self.__spec__.loader.exec_module(self)
  File "/usr/lib/python3.8/site-packages/mercurial/encoding.py", line 41, in 
<module>
    charencode = policy.importmod('charencode')
  File "/usr/lib/python3.8/site-packages/mercurial/policy.py", line 116, in 
importmod
    mod = _importfrom(pn, mn)
  File "/usr/lib/python3.8/site-packages/mercurial/policy.py", line 71, in 
_importfrom
    raise ImportError('cannot import name %s' % modname)
ImportError: cannot import name parsers

Ken

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