tests/test-gendoc-el.t

policy: add helper to import cext/pure module

These functions are sysstr API since __import__() and getattr() hate byte
strings on Python 3.

There's a minor BC, which is ImportError will be raised if invalid
HGMODULEPOLICY is specified. I think this is more desired behavior.

We're planning to add strict checking for C API compatibility. This patch
includes the stub for it.

#require docutils gettext

  $ $TESTDIR/check-gendoc el
  checking for parse errors