The goal of this work is to apply annotations in `jsonschema._format`
which match the new additions to the type stubs recently added to
typeshed.

In short, it defines a `_FormatCheckCallable` as a callable --
typically function -- which takes any object and returns a bool.
All of the `is_*` functions are `_FormatCheckCallable`s, and the
decorators are carefully annotated as being type-preserving using a
typevar.

Because the returned objects from these functions are not always
bools, this changeset now calls `bool()` on those which are not (e.g.
`ipaddress.IPv4Address`). This is really just an explicit form of the
check which is going to happen in `conforms` and `check` anyway, so
there's no significant new cost. The advantage of this is that we have
documented (via the annotations) what a format check function is
supposed to do: it returns a bool. We could equally well return `Any`
from these functions, relying on `__bool__`, but this could confuse
new contributors and users.

One unfortunate side effect of these changes is that
`FormatChecker.cls_checks` needs to be expanded into a full duplicate
of `FormatChecker.checks`. `mypy` isn't able to properly understand
what `cls_checks = classmethod(checks)` does -- this is part of a
class of semi-sophisticated callable and method manipulations that are
known to be problematic for `mypy`. In order to get the annotations
correct, the simplest solutions are either to annotate it explicitly
(cast or type comment) or to expand it as this changeset has done.

* main:
  Slightly speed up pip installs by skipping the version check in CI.
  Remove the now-unused MANIFEST.in.
  v4.6.0 -> CHANGELOG
  Ignore the badge URLs, they seem super unreliable from CI.
  Ignore a deprecation warning coming from pkg_resources on 3.11
  Add basic CONTRIBUTING guidelines.
  Make project.urls be valid URLs.
  Combine the CI and packaging workflows.
  Let RTD be authoritative about what the default doc version is.
  Re-enable Python 3.11 testing in CI.
  Modernize the packaging setup via PEP 621 and Hatch.
  Update various GHA versions.
  Update docs requirements.
  Revert "Merge pull request python-jsonschema#954 from ssbarnea/fix/py.typed"