Update pydata-sphinx-theme to v0.9.0

This also starts using python3-venv (in place of pip install --user).

Evidently pip is confused by system-installed "Pygments" package
and produced the following exception on GitHub Actions:

    'HtmlFormatter' object has no attribute 'get_linenos_style_defs'

This can be avoided by running pip in a virtual environment.

Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>

Author: fujimotos

.github/workflows/build.yaml

@@ -9,8 +9,10 @@ jobs:
       - uses: actions/checkout@v2
       - name: Build
         run: |
-          sudo apt install python3-pip
-          python3 -m pip install -U --user Sphinx pydata-sphinx-theme==0.8.1
+          sudo apt install python3-pip python3-venv
+          python3 -m venv venv
+          source ./venv/bin/activate
+          python3 -m pip install -U Sphinx pydata-sphinx-theme
           make docs
       - name: Git Push
         run: |