Use `ch` instead of `self` for pychan methods. This aligns with how
(py)chan objects are currently denoted in (py)select and will reduce the
difference when chan code is moved into libgolang. We are sticking to Go
convention here.

After the code with channels implementation is moved into libgolang, the
rest bits in golang.pyx will be changed to refer to pychan objects as
pych for clarity.