Pychan provides __eq__ (see 2c8063f4 "*: Channels must be compared by
==, not by "is" even for nilchan"), but does not provide __ne__. At the
same time in 17798442 (golang: Expose error at Py level) we had to
define both pyerror.__eq__ and pyerror.__ne__ because without __ne__
pyerror != pyerror was not working correctly.

As it turns out pychan != pychan already works ok, because pychan does
not have base class and for that case cython automatically generates
__ne__ based on __eq__:

    https://github.com/cython/cython/blob/0.29.14-629-ga73815042/Cython/Compiler/ModuleNode.py#L1963-L1976
    https://github.com/cython/cython/commit/b75d2942afab

Add corresponding comment and extend tests to make sure it is indeed so.