Fix ensuredir() in case of pre-existing file

[jdemeyer]

Subject: sphinx.util.osutil.ensuredir() should give an error when an ordinary file (not directory) exists with the given name.

Feature or Bugfix

• Bugfix

Purpose

I'm currently in the process of debugging an issue with SageMath when upgrading from Sphinx 1.7 to Sphinx 1.8. I get a warning [Errno 20] Not a directory while copying static files due to #5541 which causes the static "directory" to be an ordinary file. This should be detected earlier by ensuredir().

The current pull request also changes the philosophy of the test: we don't care about the process (why did it fail?) but we care about the result (we want a directory with the given name). By not using errno, it is very likely also more portable, which is an additional advantage.

[codecov]

Codecov Report

Merging #5548 into 1.8 will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##              1.8    #5548   +/-   ##
=======================================
  Coverage   82.06%   82.06%           
=======================================
  Files         306      306           
  Lines       40366    40366           
  Branches     6238     6238           
=======================================
  Hits        33127    33127           
  Misses       5856     5856           
  Partials     1383     1383

Impacted Files	Coverage Δ
sphinx/util/osutil.py	70.49% <0%> (ø)	⬆️

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[tk0miya]

Merged. Thank you for your contribution!