Support PURE_PYTHON

[jamadden]

Currently, the C extension is unconditionally imported:

zope.i18nmessageid/src/zope/i18nmessageid/message.py

Lines 97 to 100 in 1d6cc94

	try:
	from ._zope_i18nmessageid_message import Message
	except ImportError: # pragma: no cover
	pass

This makes it difficult to test the Python implementation on CPython; you have to remove the dynamic library.

Most Zope projects that have a C extension respect a PURE_PYTHON=1 environment variable to disable importing the C extension. This project should as well.

(See #27 (comment))