Pin python libraries

[Bckempa]

Use pinned requirements.txt and an additional constraints.txt file to ensure pip constructs a viable build environment. The upstream version (see [1]) partially depends on the distribution repositories for versioning via packages like python3-doc8 but I went though and pinned everything in the pip files instead to create a single source of truth independent of system packages.

Based on [1] by @clalancette upstream in ros2/ros2_documentation

Fixes #21

[1] ros2/ros2_documentation@e43347d

[Bckempa]

I've tested this by building the doc site with both the repo's own dockerfile and an externally managed virtualenv using Python 3.10.13 on Ubuntu 22.04. It appears to fix both the library version resolution issue as well as the Sphinx runtime exception described in #21

[ivanperez-keera]

Thanks @Bckempa . I'll review this tomorrow in detail.

A couple of changes that I can already propose are:

• Squash the commits (the second really belongs with the first in this case). If you want to split them, then I'd say that the requirements.txt goes in one commit (maybe with the constraints.txt), and the changes to the CI job go in another. But in this case, I'd accept a squash too.

• Reference the issue number you're addressing at the end of the summary line of the commit message, as in:

Pin python libraries (#21).

<rest of the message>

[Bckempa]

Thanks for the reminder, should be fixed.

[ivanperez-keera]

I can confirm that this allows me to build the image correctly.