context: Move code to pyx

In preparation to start migrating, at least partially, context
functionality to nogil mode, first, similarly to package time (see
32f34607 "time: Move code to pyx"), move the code from context.py to
_context.pyx.

This is straight code movement + associated  setup.py & co adjusts.

We don't move just to context.pyx (note no _ prefix), since we will need
to continue distinguishing pyx/nogil from py objects/functions.

golang/.gitignore

+/_context.cpp
 /_golang.cpp
 /_golang_test.cpp
 /_sync.cpp

golang/context_test.py

@@ -20,13 +20,13 @@
 
 from __future__ import print_function, absolute_import
 
-from golang import context, time, nilchan
-from golang.context import _ready as ready
+from golang import context, _context, time, nilchan
+from golang._context import _ready as ready
 from golang.time_test import dt
 
 # assertCtx asserts on state of _BaseCtx*
 def assertCtx(ctx, children, deadline=None, err=None, done=False):
-    assert isinstance(ctx, context._BaseCtx)
+    assert isinstance(ctx, _context._BaseCtx)
     assert ctx.deadline() == deadline
     assert ctx.err() is err
     assert ready(ctx.done()) == done

setup.py

@@ -214,6 +214,9 @@ setup(
                          'golang/runtime/libgolang_test_c.c',
                          'golang/runtime/libgolang_test.cpp']),
 
+                    Ext('golang._context',
+                        ['golang/_context.pyx']),
+
                     Ext('golang._sync',
                         ['golang/_sync.pyx']),
                     Ext('golang._sync_test',