Updated all requirements-doc (#58)

The requirements-doc.txt had to be updated since building docs fails on
readthedocs.

Author: mristin

requirements-doc.txt

@@ -1,7 +1,7 @@
-sphinx>=1.7.7,<2
-sphinx-autodoc-typehints>=1.3.0,<2
-sphinx-icontract>=1.0.0
-sphinx-rtd-theme>=0.4.1,<1
+sphinx>=3.2.1,<4
+sphinx-autodoc-typehints>=1.11.0,<2
+sphinx-icontract>=2.0.0
+sphinx-rtd-theme>=0.5.0,<1
 
 # We need to pin icontract version since readthedocs for some reason picks an older version
 # which could not be installed on their remote servers.