Update Sphinx and Read the Docs dependency

When pushing our docs to GitHub pages in the last days we were
surprised when some parts were not rendered correctly. We have
discovered that this has something to do with the version of Read the
docs theme we're using (more info is available her in this issue:
readthedocs/sphinx_rtd_theme#1115), so we had
to upgrade sphix-rtd-theme package to 0.5.2 and we also updated Sphinx
to the latest 3.5.3 version.

Pipfile

@@ -16,8 +16,8 @@ selinux = "==0.2.1"
 setuptools = "*"
 wheel = "*"
 twine = ">=3.0.0"
-sphinx = "==3.3.1"
-sphinx-rtd-theme = "==0.5.0"
+sphinx = "==3.5.3"
+sphinx-rtd-theme = "==0.5.2"
 sphinx-tabs = "==2.1.0"
 sphinx-argparse = "==0.2.5"
 sphinx-copybutton = "==0.3.1"