Fix a typo for version number in requirements-dev.txt (#759)

The current version of requirements-dev.txt includes sphinx_rtd_theme=0.4.3. 
To use pip install -r requirements-dev.txt, it requires the version number to be 
specified using ==. I added the second = in the pull request.

Author: willschlitzer

requirements-dev.txt

@@ -12,6 +12,6 @@ isort>=5
 pylint
 flake8
 sphinx
-sphinx_rtd_theme=0.4.3
+sphinx_rtd_theme==0.4.3
 sphinx-gallery
 nbsphinx