Update sphinx requirement from ~=3.1.1 to ~=3.1.2 (#614)

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

requirements-dev.txt

@@ -8,5 +8,5 @@ ipython ~=7.16.1
 matplotlib ~=3.3.0
 numpydoc ~=1.0.0
 requests-mock ~=1.8.0
-sphinx ~=3.1.1
+sphinx ~=3.1.2
 sphinx_rtd_theme ~=0.5.0