py-sphinx: update to 1.6.4

Release 1.6.4:

Features added
* 3926: Add autodoc_warningiserror to suppress the behavior of -W option during importing target modules on autodoc

Bugs fixed
* 3924: docname lost after dynamically parsing RST in extension
* 3946: Typo in sphinx.sty (this was a bug with no effect in default context)
* pep: and :rfc: does not supports default-role directive (refs: 3960)
* 3960: default_role = ���guilabel��� not functioning
* Missing texinputs_win/Makefile to be used in latexpdf builder on windows.
* 4026: nature: Fix macOS Safari scrollbar color
* 3877: Fix for C++ multiline signatures.
* 4006: Fix crash on parallel build
* 3969: private instance attributes causes AttributeError
* 4041: C++, remove extra name linking in function pointers.
* 4038: C, add missing documentation of member role.
* 4044: An empty multicolumn cell causes extra row height in PDF output
* 4049: Fix typo in output of sphinx-build -h
* 4062: hashlib.sha1() must take bytes, not unicode on Python 3
* Avoid indent after index entries in latex (refs: 4066)
* 4070: crashes when the warning message contains format strings
* 4067: Return non-zero exit status when make subprocess fails
* 4055: graphviz: the :align: option does not work for SVG output
* 4055: graphviz: the :align: center option does not work for latex output
* 4051: warn() function for HTML theme outputs ���None��� string

(adam)