Shellous

------

- [API] Add `cancel` and `send_signal` methods to `Runner` class.
- [LATERAL] Add some type annotations internally.

------

- [API] Remove DeprecationWarnings. Literal constants are no longer supported in redirections.
- [API] Redirecting stdout/stderr to a string/bytes file name is no longer supported. (Use `pathlib.Path`)
- [API] Remove the deprecated `context()` function.
- [LATERAL] Improve test reliability by increasing some timeouts slightly.

------

- [LATERAL] Update `immutables` dependency version.
- [LATERAL] Include ThreadStrategy in code coverage tests.

------

- [API] In the future, refer to redirection constants using `sh`: `sh.DEVNULL`, `sh.STDOUT`, `sh.INHERIT`.
- [API] Redirecting output to a str or bytes object raises a DeprecationWarning; use a Path object instead.
- [API] The `context()` function now raises a DeprecationWarning.
- [API] Using literal constants None, ..., () and 1 in redirection operators now raises a DeprecationWarning.
- [API] Rename `incomplete_result` setting to `catch_cancelled_error`.
- [LATERAL] Work-around test that is failing on Alpine linux (241).

------

- [API] Add `sh` global execution context and update documentation to use it instead of `context()`.
- [LATERAL] Update `immutables` dependency version.
- [LATERAL] Add continuous integration support for pypy-3.9.

------

- [LATERAL] Clean up DefaultChildWatcher implementation. Keep references to created tasks.
- [LATERAL] Use a contextvar to control whether to deactivate child watcher for certain processes.
- [BUGFIX] Use `.name` to convert signal enums to strings rather than `str()`.