Add cron job to GitHub Actions and expand extras_require

[kitchoi]

This PR adds a cron job to the GitHub Actions workflow.

Different from the cron job currently setup on Travis which uses EDM, this cron job is setup to install everything from pip.

This exercise also helps flush out missing dependency declaration in setup.py, which isn't used by ci/edmtool.py. With that, the extras_require declarations for optional dependencies is expanded here so that CI setup can call pip install .[flag,...]. This will further benefit package distributors who will be looking for these declarations when they build enable.

As the names of the optional dependencies are tied to the cron job setup, I keep these two changes in the same PR.

Since tests on optional features are skipped if the optional dependency is absent (apart from the ones on SVG, see #543), I checked the CI job output to look out for unexpected skips.

Note to reviewer

The current trigger is on: push, for testing purposes. Once the reviewer is happy with the change, I will switch the event trigger to:

on:
  schedule:
    - cron:  '0 0 * * 5'

and request one more review again before merging.

[kitchoi]

I tried the combination of Python 3.9 and Ubuntu 20, but there aren't a prebuilt wheel for wxPython, which means we will have to build from source (take a long time!). But then I haven't tried bumping Python version again for Ubuntu 18.

I did not use Ubuntu 16, as an attempt to be as close to the latest as possible, and also to be different from the per-PR CI job (using Ubuntu 16) to expand coverage.

[rahulporuri]

LGTM with one question.

[rahulporuri]

still LGTM

[kitchoi]

Thank you @rahulporuri
As promised, the on trigger is updated to 'schedule', does this still look okay to merge?

[rahulporuri]

still LGTM

[kitchoi]

Thank you!