golang: Make sure that pychan != pychan works correctly

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.

golang/_golang.pyx

@@ -298,6 +298,8 @@ cdef class pychan:
     # pychan == pychan
     def __hash__(pychan pych):
         return <Py_hash_t>pych._ch
+    # NOTE __ne__ not needed: pychan does not have base class and for that
+    # case cython automatically generates __ne__ based on __eq__.
     def __eq__(pychan a, object rhs):
         if not isinstance(rhs, pychan):
             return False

golang/golang_test.py

@@ -889,24 +889,27 @@ def test_chan_dtype_misc(dtype):
         assert nilch is nilchan
 
     assert hash(nilch) == hash(nilchan)
-    assert nilch == nilch                   # nil[X] == nil[X]
-    assert nilch == nilchan                 # nil[X] == nil[*]
-    assert nilchan == nilch                 # nil[*] == nil[X]
+    assert      (nilch == nilch)            # nil[X] == nil[X]
+    assert not  (nilch != nilch)
+    assert      (nilch == nilchan)          # nil[X] == nil[*]
+    assert not  (nilch != nilchan)
+    assert      (nilchan == nilch)          # nil[*] == nil[X]
+    assert not  (nilchan != nilch)
 
     # channels can be compared, different channels differ
     assert nilch != None    # just in case
     ch1 = chan(dtype=dtype)
     ch2 = chan(dtype=dtype)
     ch3 = chan()
-    assert ch1 != ch2;  assert ch1 == ch1
-    assert ch1 != ch3;  assert ch2 == ch2
-    assert ch2 != ch3;  assert ch3 == ch3
+    assert ch1 != ch2;  assert not (ch1 == ch2);  assert ch1 == ch1; assert not (ch1 != ch1)
+    assert ch1 != ch3;  assert not (ch1 == ch3);  assert ch2 == ch2; assert not (ch2 != ch2)
+    assert ch2 != ch3;  assert not (ch2 == ch3);  assert ch3 == ch3; assert not (ch3 != ch3)
     assert hash(nilch) != hash(ch1)
     assert hash(nilch) != hash(ch2)
     assert hash(nilch) != hash(ch3)
-    assert nilch != ch1
-    assert nilch != ch2
-    assert nilch != ch3
+    assert nilch != ch1;  assert not (nilch == ch1)
+    assert nilch != ch2;  assert not (nilch == ch2)
+    assert nilch != ch3;  assert not (nilch == ch3)
 
     # .nil on chan instance     XXX doesn't work (yet ?)
     """