hgdemandimport: bypass demandimport for _ast module (issue6407)

This is broken on Python 3.9rc1, and while it sounds like there may be
a fix in Python, we probably also should have this workaround in place
in hg. See the bug for more details (including on bugs at redhat and
b.p.o).

Differential Revision: https://phab.mercurial-scm.org/D9004

hgdemandimport/__init__.py

@@ -46,6 +46,7 b' IGNORES = {'
     # setuptools' pkg_resources.py expects "from __main__ import x" to
     # raise ImportError if x not defined
     '__main__',
+    '_ast',  # https://bugs.python.org/issue41631
     '_ssl',  # conditional imports in the stdlib, issue1964
     '_sre',  # issue4920
     'rfc822',