Slipcover

This version makes no visible changes to Slipcover when used as a coverage analyzer.
It starts to improve Slipcover's `import` interface, making it easier to use for coverage-guided fuzzing.

This release:
- adds `--skip-covered` to skip fully (100%) covered files when printing coverage, suggested by LilyFoote;
- adds `--fail-under T` to have Slicover fail with a return code of 2 if the overall coverage is less than `T`, suggested by rec;
- fixes a division by 0 when printing out coverage for empty files on Python 3.11; thank you, diceroll123!

This release:
- fixes failing tests with Python 3.11;
- adds wheels for Python 3.11;
- adds an experimental option, `--immediate`, which essentially implements on-stack replacement by ignoring Python's rule that code objects should be immutable. Using it further reduces overhead, as Slipcover need not generate new code objects or update references to it. Use at your own risk... this is really only intended for research purposes;
- fixes an include file, ironically, not being included into the source distribution;

This release:
- fixes a bug causing Slipcover to move functions' docstrings when doing branch coverage, so that they could no longer be found;
- advances version to 0.2.0 given the support for branch coverage;

This release:
- adds branch coverage measuring support to slipcover; simply run it with `--branch` to get it.
- adds a "meta" item identify the source of the coverage information, as well as per-file and global summaries to the JSON output (issue 2).

This release:
- fixes a packaging issue omitting Python 3.11 from classifiers;
- acknowledges pypy support, contributed by cfbolz, and actually added in v0.1.7;