Slurry

* Add support for python 3.10, remove support for python 3.7.
* Bump trio to 0.21.0
* Minor documentation fixes.

* Added Metronome, which is a repeater section similar to Repeat, however the timing is based on the wall clock and
receiving new items doesn't reset the timer.
* Added InsertValue, which sends a single item once, and then passes through any received items unmodified.
* Updated all dependencies.
* Support for python 3.10 was intended for this version, but tests are failing at the moment, due to a build issue.
Therefore 3.10 support is postponed for now.

* Added SkipWhile section, which skips items until a predicate evaluates to false.
* Update all dependencies. Minimum python version bumped to 3.7.

* Documentation has been further reworked and refined. As this version is feature complete for my personal use, and
has good documentation, we are good to release as v1.0.

* Update to trio 0.18

* Fix unhandled StopAsyncIteration in Skip section.