context: Export PyContext for cimport

For example wendelin.core wants to accept Python-level context,
transform it into C-level context and pass to pyx/nogil code.

golang/_context.pxd

@@ -72,3 +72,14 @@ cdef extern from "golang/context.h" namespace "golang::context" nogil:
 
     # for testing
     cxx.set[Context] _tctxchildren(Context ctx)
+
+
+# ---- python bits ----
+
+from golang cimport pychan
+from cython cimport final
+
+@final
+cdef class PyContext:
+    cdef Context  ctx
+    cdef pychan   _pydone # pychan wrapping ctx.done()

golang/_context.pyx

@@ -48,8 +48,6 @@ cdef PyContext _newPyCtx(Context ctx):
 # key -> value dict.
 @final
 cdef class PyContext:
-    cdef Context  ctx
-    cdef pychan   _pydone # pychan wrapping ctx.done()
 
     def __cinit__(PyContext pyctx, object bywho):
         if bywho is not _frompyx: