C domain: Fix namespace-pop context

[ferdnyc]

The ref_context key is c:parent_key, not cp:parent_key.

Subject: Correct a ref_context key in CNamespacePop

Feature or Bugfix

• Bugfix

Purpose

• Correct a ref_context item named cp:parent_key, which the CNamespacePop directive was seetting when restoring the context.

Detail

• The erroneous key here was the only instance in the entire codebase of that name being used. All others are c:parent_key.
• This appears to have likely been a typo, made when converting the code from the C++ domain and editing the key down from cpp:parent_key.

[ferdnyc]

A Windows failure, once again, in the unrelated test tests/test_intl/test_intl.py::test_gettext_dont_rebuild_mo.