Add watchfiles as dependency

[radiac]

The watchfiles dependency used in server.py is missing from requirements.

Since the changes 5 days ago, with a clean install, you get the following error:

$ sphinx-autobuild
Traceback (most recent call last):
  File "/.../sphinx-autobuild", line 5, in <module>
    from sphinx_autobuild.__main__ import main
  File "/.../sphinx_autobuild/__main__.py", line 22, in <module>
    from sphinx_autobuild.server import RebuildServer
  File "/.../sphinx_autobuild/server.py", line 8, in <module>
    import watchfiles
ModuleNotFoundError: No module named 'watchfiles'

After installing watchfiles the project runs successfully. This PR adds the package as a dependency in pyproject.toml.

[hugovk]

@AA-Turner @pradyunsg Please could we have a bugfix release?

Otherwise we'll need to either pin to the old version or add watchfiles as a temporary dependency to CPython docs (plus backports) and the devguide and PEPs.

Thank you!

[pradyunsg]

@AA-Turner clicked the relevant buttons and made it happen. :)

[hugovk]

Confirmed working now, thanks all!