Add requirements.txt

Move the list of Python packages we need with their versions specifiers
into a new requirements.txt file, which we can pass directly to pip3 for
installation.

Update the README and the CI accordingly.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

Author: vstehle

.github/workflows/main.yaml

@@ -81,7 +81,8 @@ jobs:
 
       - name: 'Install Python required packages'
         run: |
-          pip3 install --user --break-system-packages mako typing Sphinx~=8.1
+          pip3 install --user --break-system-packages \
+            --requirement requirements.txt
 
       - name: 'Check'
         run: make check-local

README.rst

@@ -56,7 +56,7 @@ If the version of python-sphinx installed is too old, then an additional
 new version can be installed with the Python package installer::
 
   $ apt-get install python3-pip
-  $ pip3 install --user --upgrade Sphinx
+  $ pip3 install --user --upgrade --requirement requirements.txt
   $ export SPHINXBUILD=~/.local/bin/sphinx-build
 
 Export SPHINXBUILD (see above) if Sphinx was installed with pip3 --user,

requirements.txt

@@ -0,0 +1,5 @@
+# This is the list of Python packages we need for EBBR.
+# Install with pip3 --requirement.
+Sphinx ~= 8.1
+mako
+typing