Add _ast to hgdemandimport ignore list

I'm not sure whether to call this a workaround or a fix;
that depends on how the CPython issue is solved.

Works around: BZ#1871992

hgdemandimport_ast.patch

@@ -0,0 +1,11 @@
+diff -U3 -r mercurial-5.4-orig/hgdemandimport/__init__.py mercurial-5.4/hgdemandimport/__init__.py
+--- mercurial-5.4-orig/hgdemandimport/__init__.py	2020-05-01 18:17:29.000000000 +0200
++++ mercurial-5.4/hgdemandimport/__init__.py	2020-09-01 18:04:40.540165952 +0200
+@@ -56,6 +56,7 @@
+     '__builtin__',
+     'builtins',
+     'urwid.command_map',  # for pudb
++    '_ast', # https://bugzilla.redhat.com/show_bug.cgi?id=1871992
+ }
+ 
+ _pypy = '__pypy__' in sys.builtin_module_names

mercurial.spec

@@ -1,7 +1,7 @@
 Summary: Mercurial -- a distributed SCM
 Name: mercurial
 Version: 5.4
-Release: 3%{?dist}
+Release: 4%{?dist}
 
 # Release: 1.rc1%%{?dist}
 
@@ -14,6 +14,7 @@ URL: http://www.selenic.com/mercurial/
 Source0: http://www.selenic.com/mercurial/release/%{name}-%{upstreamversion}.tar.gz
 Source1: mercurial-site-start.el
 Patch2: 0001-setup-hg3.patch
+Patch3: hgdemandimport_ast.patch
 BuildRequires: python2-devel python3-devel bash-completion
 BuildRequires: emacs-nox emacs-el pkgconfig gettext python3-docutils
 BuildRequires: gcc
@@ -133,6 +134,8 @@ Locales for mercurial.
 # sed -ri 's|python\b|python2|' %{_builddir}/%{name}-%{version}/Makefile %{_builddir}/%{name}-%{version}/doc/Makefile
 %patch2 -p1 -b .create_hg3
 
+%patch3 -p1
+
 %build
 # copy hg to hg2/hg3 to be able to create /usr/bin/hg(2|3) script
 cp -a hg hg2
@@ -332,6 +335,10 @@ fi
 
 
 %changelog
+* Wed Sep 02 2020 Petr Viktorin <pviktori@redhat.com> - 5.4-4
+- Add _ast to hgdemandimport ignore list
+  Works around: BZ#1871992
+
 * Mon Aug 10 2020 Petr Stodulka <pstodulk@redhat.com> - 5.4-3
 - Fix upgrade from previous mercurial 4.9 causing broken alternatives for
   mercurial