Release 0.15

[awestlake87]

Consolidating changes from #52 and #54 (maybe #43 as well) to prepare for release 0.15

TODO:

• [x] Add fallback for contextvars in Python 3.6
• [x] Change TaskLocals from #[non_exhaustive] to private fields with public accessors
• [x] Deprecate *_with_loop variants in favor of *_with_locals
• [x] Deprecate cancellable_* variants
• [x] ? Merge in #43 and update *_with_locals variants to have generic returns
• [x] Remove 0.14 deprecations
• [x] Docs
  • [x] Include contextvars in discussion around Event Loop References
  • [x] Document limitation around task locals and synchronous python functions called from a Rust thread
  • [x] Ensure that docs mention that cancellable futures are now the default behaviour
  • [x] Document that cancellable_* and *_with_loop variants will be removed in 0.16
  • [x] v0.14 -> v0.15 migration guide (*_with_loop deprecation and generic returns)
  • [x] Open PyO3 guide PR for 0.15 changes if necessary
  • At a glance, I believe it's not necessary this time. The only changes I could see were simplifying Ok(Python::with_gil(|py| py.None())) to Ok(()).
  • [x] Full review
  • I'll do a full review / link check once I merge it into master and see the API docs on GitHub pages

[jnicholls]

Love it, thanks for this awesome crate.

[awestlake87]

Thanks @jnicholls! I'm hoping to get this PR wrapped up and released as 0.15 soon