Docs: clarify versioning expectations and back-compatibility

[amyreese]

[thatch]

Some off-the-cuff ideas, centered around the definition required if we wanted to adopt https://github.com/meanver/meanver/

It is our intent to:

1. Bump the post version (and yank the original, depending on how severe) for:

   • packaging issues, such as missing files in the sdist, unintended classifiers

2. Bump the patch version for:

   • garden-variety bugs, such as those where the previous behavior was an exception (such as KeyError while sorting duplicate imports #194)
   • any bug that produces output so mangled it would not have been accepted by human review (such as producing mojibake)

   Additionally, if the bug causes data loss, we will also yank affected releases (best effort), but the timing of the yank is not defined here. The spirit of this is to prevent people from installing known-bad versions, but if people pin then yanking is a sign, not a cop.

3. Bump the minor version for:

   • new features that may change the output (at the byte level) for greenfield sorting, but produce output that can then be sorted with previous versions without producing additional changes (such as Feature: split basic imports #140)
   • anything that includes a new config option (although the defaults for those config options must keep previous behavior); if a consumer project relies on a certain config option, it is on them to constrain to >= the version where that was added
   • any new DeprecationWarning
   • changing strings intended for humans (e.g. Result.warnings) or exception classes (e.g. Result.exception, or its specific values or hierarchy) which are not strongly considered "part of the API" [TODO: what is the API]

4. Bump the major for:

   • any formatting change that fails "test_backwards_compatibility" on CI, which formats some ever-growing piece of code with the development version, then downgrades to the earliest known release from that major series and confims no diff
   • any intentional change to picking pyproject.toml or inferring first-party names
   • any API removal. [TODO: what is the API]

   Additionally, within reason we will yank releases that should have bumped the major but didn't, once informed.

5. Note that we assume our deps follow reasonable versioning practices (and since several of them are us with another hat on, this is believable). In particular:

   • We rely on trailrunner and constrain to a major version, under the assumption that if trailrunner ever has a non-silly case where it would choose a different pyproject.toml that it would bump the major. Examples of silly cases are UNC paths on windows, bind-mounted path loops on linux.
   • We rely on stdlibs to classify top-level names in the stdlib section or not. This is date-versioned currently, and in general we want the updates like Update stdlib names for 3.11 beta omnilib/stdlibs#25, even though that can cause formatting changes in uncommon cases. If you are sensitive to this, consider pinning stdlibs as well as your usort (and probably black).