docs: Force Sphinx version higher or equal to 3.0

Signed-off-by: Robert Winkler <rwinkler@antmicro.com>

docs/conf.py

diff --git a/docs/conf.py b/docs/conf.py
index d9ac409..d302154 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -39,7 +39,7 @@
 
 # If your documentation needs a minimal Sphinx version, state it here.
 #
-# needs_sphinx = '1.0'
+needs_sphinx = '3.0'
 
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom

docs/requirements.txt

diff --git a/docs/requirements.txt b/docs/requirements.txt
index 05e7456..622ee48 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,8 +1,7 @@
 sphinx_materialdesign_theme
 
 docutils
-# Disabling direct sphinx because of https://github.com/pypa/pip/issues/9031
-# sphinx
+sphinx>=3.0
 sphinx-autobuild
 
 breathe