A backport of the Python 3.6 inspect module to Python 2.7-3.5.

This module was based on the code of the inspect module and its tests in the Python 3.6 standard library and modified to pass tests on earlier Python versions.

I have tried to follow these rules in writing the code:

• Minimize changes relative to the 3.6 source. This should make backporting of future changes easier.
• No support for old-style classes. Python 2 is hard enough without them.
• All functions should be available on all versions. inspect2.iscoroutinefunction() just always returns False on Python versions that don't have async def.