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