Remove tox version cap (#10730)

The recent release of pyproject-api 1.6.x seems to have broken
compatibility with older versions of tox. We had capped the tox version
at < 4.4.0 for some time to avoid other issues in tox. This commit
removes that cap to test if it is fixed and we can use the latest
versions of tox and pyproject-api together. If this fails CI then the
alternative approach will be to use the constraints file to cap
pyproject-api to < 1.6.0.

tools/install_ubuntu_docs_dependencies.sh

@@ -5,7 +5,7 @@
 set -e
 
 python -m pip install --upgrade pip setuptools wheel
-python -m pip install --upgrade "tox<4.4.0"
+python -m pip install --upgrade tox
 
 sudo apt-get update
 sudo apt-get install -y graphviz pandoc