Make the watchfiles dependency optional

[bluetech]

trafficstars

Granian depends on the watchfiles Python package for the --reload (reload on file changes) feature. This feature is helpful for local development, but is not needed on production (at least that I know of!).

The watchfiles package brings in the following extra packages to the granian dependency closure:

• watchfiles
  • anyio
    • idna
    • sniffio

To reduce the dependencies in production, I propose making watchfiles optional:

• Move the watchfiles import to be done lazily in _serve_with_reloader.
• Remove the watchfiles from the default dependencies and move it to a granian[watch] extra instead.
• Update the docs to mention that the [watch] extra is needed for --reload to work.

I can implement this if it sounds good.

[gi0baro]

Sounds reasonable to me, I'll be happy to accept a PR for this. The only thing I would change in your plan is to name the extra reload instead of watch just to be more consistent.