fix broken bullets in toc

Due to an issue with the compatibility of sphinx-rtd-theme
and docutils, there are problems with rendering some doc elements
when using the most recently installed versions.

This is a known bug (see, among others,
readthedocs/sphinx_rtd_theme#1115).

This pins Sphinx and sphinx-rtd-theme to versions that work for
now.

Signed-off-by: Szymon Łopaciuk <szymon@lopaciuk.eu>

requirements.txt

@@ -1,4 +1,6 @@
 numpy
 scipy
 pandas
-lark
+lark
+sphinx_rtd_theme>=0.5.2
+sphinx==5.3.0