Use RTD theme both locally and on RTD

See https://docs.readthedocs.org/en/latest/theme.html#how-do-i-use-this-locally-and-on-read-the-docs

Author: ddemidov

docs/conf.py

@@ -20,6 +20,8 @@
 sys.path.append(os.path.join(os.path.dirname(__file__), '..'))
 from git_version import git_version
 
+on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
+
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
@@ -115,7 +117,8 @@
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-#html_theme = 'sphinx_rtd_theme'
+if not on_rtd:
+    html_theme = 'sphinx_rtd_theme'
 
 def setup(app):
     # overrides for wide tables in RTD theme