context: Provide way to create PyContext from raw Context

sync.PyWorkGroup will need to wrap raw Context into PyContext for
spawned workers.

golang/_context.pxd

@@ -83,3 +83,7 @@ from cython cimport final
 cdef class PyContext:
     cdef Context  ctx
     cdef pychan   _pydone # pychan wrapping ctx.done()
+
+    # PyContext.from_ctx returns PyContext wrapping pyx/nogil-level Context ctx.
+    @staticmethod
+    cdef PyContext from_ctx (Context ctx)

golang/_context.pyx

@@ -49,6 +49,10 @@ cdef PyContext _newPyCtx(Context ctx):
 @final
 cdef class PyContext:
 
+    @staticmethod
+    cdef PyContext from_ctx(Context ctx):
+        return _newPyCtx(ctx)
+
     def __cinit__(PyContext pyctx, object bywho):
         if bywho is not _frompyx:
             raise AssertionError("Context must not be instantiated by user")