Update sphinx to 1.6.4

[pyup-bot]

There's a new version of sphinx available.
You are currently using 1.6.3. I have updated it to 1.6.4

These links might come in handy: PyPI | Changelog | Homepage

Changelog

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

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖