demandimport: add _imp to ignore list

Mozilla is seeing an issue with demand importing of _imp
failing in pkg_resources/__init__.py:fixup_namespace_packages.
It strangely only reproduces when using a modern version of
setuptools/pip in certain scenarios. Adding _imp to the demand import
ignore list seems to make the problem go away.

mercurial/demandimport.py

@@ -230,6 +230,8 b' def _demandimport(name, globals=None, lo'
 ignore = [
     '__future__',
     '_hashlib',
+    # ImportError during pkg_resources/__init__.py:fixup_namespace_package
+    '_imp',
     '_xmlplus',
     'fcntl',
     'win32com.gen_py',