DOC: Mark pytest module

Pytest document currently does not index the top-level package name
`pytest`, which causes some trouble when building documentation that
cross-refers to the pytest package via ``:mod:`pytest` ``.

doc/en/reference.rst

@@ -3,6 +3,8 @@
 API Reference
 =============
 
+.. module:: pytest
+
 This page contains the full reference to pytest's API.
 
 .. contents::