Add standard-imghdr to requirements

[tgaspar]

Summary

This PR adds the standard-imghdr dependency to requirements.txt to make build compatible in newer (>3.13) Python environments while at the same time abiding to the sphinx==5.0.

When building the Sphinx documentation with the sphinxdoc/sphinx Docker image (same happens when using the latest Python image) I noticed errors stating that a Python module was missing:

Could not import extension sphinx.builders.epub3 (exception: No module named 'imghdr')

This error does not appear in CI because the Github build pipeline uses an older version of Python:

    - uses: actions/setup-python@v6
      with:
        python-version: '3.8'
        cache: 'pip'

The module imghdr has been deprecated in Python 3.11 and completely removed in Python 3.13 (see Removed Modules and PEP 0594.

Reproducing the error

To reproduce the issue:

Navigate to the docs directory and start the Python (or Sphinx) Docker container (for Sphinx replace python:3 with sphinxdoc/sphinx):

docker run -it -v `pwd`:/docs --rm python:3 /bin/bash

Then, inside the Docker, install the dependencies:

cd /docs; pip3 install -r requirements.txt

Finally, try building the documentation with (you're already in the docs directory):

make html

The result should be the following error

Running Sphinx v5.0.0

Extension error:
Could not import extension sphinx.builders.epub3 (exception: No module named 'imghdr')
make: *** [Makefile:26: html] Error 2

Then, manually try manually installing standard-imghdr:

pip3 install standard-imghdr

Try rebuilding now and the error is gone.

Backwards compatibility

To test backwards compatibility, checkout this branch that adds standard-imghdr to the requirements.txt file. Then, repeat the process described above with an older version of Python. For instance, use the image for Python 3.8:

docker run -it -v `pwd`:/docs --rm python:3.8 /bin/bash

Additional note

This error arises mainly due to defining relatively old versions of both Sphinx (==5.0) and Python (==3.8). I also tested building the documentation with a newer version of Sphinx and did not notice any build errors. However, I think that adding standard-imghdr to the requirements.txt is a smaller change and less likely to disrupt anything.