Update Tox settings for easier use & better performance

[aitorres]

What's changed

This PR updates Tox's settings in order to:

• Consider only paths and files related to the project while running code style / lint / type checks, instead of the whole project (which could potentially include a virtual environment) (as reported in Ignore virtualenvs when running lints (flake8, black, etc) with tox #191).
• Remove the py37 constraint while setting up the environment, in order to let users run Tox with other versions (specifically newer versions of Python) (as reported in Tox should default to current python, not py37 #189)

CONTRIBUTING.md was also updated to remove references to the lint.sh script, which was removed, and replace its usage with tox in order to achieve the same effects.

Two entries were added to the changelog, for doc and misc changes.

Solved issues

Closes #189 if merged.
Closes #191 if merged.

[callahad]

Looks good, thank you for the contribution! Two very small nits and then it should be good to merge.

[aitorres]

All done! Those two things flew past my radar 😅 fixed now.

By the way, not sure if this is the most appropriate place for this: I would like to start contributing to Matrix in general with some frequency, but don't know what's the best way to start. Is there any public, central hub / channel for contributors to communicate?

Edit: Found the rooms on Matrix :-)

[callahad]

By the way, not sure if this is the most appropriate place for this: I would like to start contributing to Matrix in general with some frequency, but don't know what's the best way to start. Is there any public, central hub / channel for contributors to communicate?

Edit: Found the rooms on Matrix :-)

Oh yay! Please do poke around, get a feel for how Matrix works as a platform, etc. and feel free to ask us any questions you might have. I'm usually in the Synapse Dev and Sygnal rooms.

[callahad]

Hooray! Thank you so much.