[mod] 3.8: Install mypy, pylint, and sphinx via pip

- clients/python/duniterpy#118
- To speed-up the jobs

3.8/Dockerfile

@@ -6,4 +6,4 @@ apt install --yes libsodium23 make && \
 rm -rf /var/lib/apt/lists
 
 # Install Poetry
-RUN pip3 install poetry black
+RUN pip3 install poetry black mypy pylint sphinx_rtd_theme