In a followup commit we are going to add channel types (e.g. chan of
double, chan of int, etc) and accordingly there will be several nil
channel objects, e.g. nil(dtype=int), nil(dtype=double) etc, which will
be separate python objects. Even without data types, another planned
change is to add directional channels, e.g. a channel instance that can
only send, but not receive and vice versa(*).

This way for the same underlying channel object, there can be several
pychan objects that point to it - even for nil channel - e.g. nilchan
and `niltx = nilchan.txonly()` that creates another pychan object
pointing to the same underlying nil.

Since we want all channels (of the same type) that point to the same
underlying channel to compare as same, we cannot use "is" for comparison
and have to use ==. In other words channels, both at C and Python level,
should be perceived as pointers, of which there can be multiple ones
pointing to the same location, and thus == has to be used to compare
them.

(*) see https://golang.org/ref/spec#Channel_types for details.