Correctly deal with non-existing static dir in graphviz extension

[jdemeyer]

Subject: if the _static destination directory does not exist, the file graphviz.css is copied to a file called _static instead of a file in the directory _static.

Feature or Bugfix

• Bugfix

[codecov]

Codecov Report

Merging #5549 into 1.8 will decrease coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##              1.8    #5549      +/-   ##
==========================================
- Coverage   82.06%   82.06%   -0.01%     
==========================================
  Files         306      300       -6     
  Lines       40366    40027     -339     
  Branches     6238     6184      -54     
==========================================
- Hits        33127    32848     -279     
+ Misses       5856     5803      -53     
+ Partials     1383     1376       -7

Impacted Files	Coverage Δ
sphinx/ext/graphviz.py	31.83% <50%> (ø)	⬆️
sphinx/make_mode.py
sphinx/search/__init__.py
sphinx/quickstart.py
sphinx/errors.py
sphinx/__init__.py
sphinx/apidoc.py

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[tk0miya]

Thanks!