Add more posix.unistd constants

Add posix constants for _CS_*, _PC_* and _SC_*.
Merge pull request #302 from markpeek/markpeek-posix-unistd

Add more posix.unistd constants
prevent loop cleanup code from leaking into loop body annotation
fix error output in test
avoid Py2/3 syntax incompatibility after 2to3
disable gdb test failures as long as the tests don't actually work
add __name__ and __qualname__ properties to generators
add docstrings for new generator properties
extend tests for new generator properties
fix compiler crash in CalculateQualifiedNamesTransform
Bump version.
Merge branch '0.20.x'

Conflicts:
	CHANGES.rst
	Cython/Shadow.py
changelog
be more informative about current Python version when it's too old (might be automatically selected by shebang)
merge
implement relative cimports and fix some general issues with relative imports
use explicit relative imports everywhere and enable absolute imports by default