Switch to the more modern importlib approach as it doesn't
require to modify sys.path:
https://docs.pytest.org/en/7.2.x/explanation/pythonpath.html

tox/pytest: Add testpaths and use importlib

This class variable contains a tuple of strings that
contains the names of all attributes of a Version
(like "major", "minor" etc).

In cases we need to have dynamical values, this
makes it easier to iterate.

Trivial: Introduce public Version.NAMES class variable

The current example, when used with semver 3.0.0.dev4 and pydantic
1.10.4, produces the following error:

  pydantic.errors.ConfigError: Invalid signature for validator <bound
  method Version.parse of <class '__main__.PydanticVersion'>>: (version:
  Union[str, bytes], optional_minor_and_patch: bool = False) ->
  'Version', should be: (value, values, config, field), "values",
  "config" and "field" are all optional.

This commit fixes the example in the documentation so that it works and
does not raise an error.

Fix pydantic/semver example

This was caused by __main__.py passing `sys.argv` to `cli.main()`,
which includes the path of the script in `sys.argv[0]`.
`argparse.ArgumentParser.parse_args()` parses this as the subcommand
name, causing it to throw an error.

Fix: All `python -m semver` commands fail with 'error: invalid choice:'

So that calling parse on a derived class will show correct type of
derived class

* Implement Version.is_compatible() method
* Update test cases
* Update documentation
* Rename isvalid method to is_valid to make it consistent
  with is_compatible

The result is True, if either of the following is true:

* both versions are equal, or
* both majors are equal and higher than 0. Same for both minors.
  Both pre-releases are equal, or
* both majors are equal and higher than 0. The minor of b's
  minor version is higher then a's. Both pre-releases are equal.

The algorithm does *not* check patches!

Co-authored-by: Lexi Robinson <lexi@lexi.org.uk>
Co-authored-by: Thomas Laferriere <tlaferriere@users.noreply.github.com>
Co-authored-by: Raphael Krupinski <rafalkrupinski@users.noreply.github.com>

The methods .bump_prerelease() and .bump_build() allow now
different types for the token argument:

* A string. This was already allowed and default was "rc".
  If the string is empty, we get only the raised number (without
  the "rc" part). The number starts from 1.
* None. Is the same as calling the method without any argument.

Fix #344 Allow empty string for bump method

The asterisk in "python_requires = >= 3.7.*" make the
pyproject-build command fail with this exception:

 setuptools.extern.packaging.specifiers.InvalidSpecifier: Invalid specifier: '>=3.7.*'

This fix removes the asterisk which leads to a successful build.

Fix #397: Remove asterisk in python_requires

Fix #284: implement "is compatible with" method

* Combine all dev releases together in one changelog
* Remove all changelog.d entries and integrate it into CHANGELOG
* Move dev releases to file changelog-semver3-devel.rst
* Split contributing into different sections
* Some doc polishing; use :meth: consistently for semver.Version methods
* Fix also some doc bugs
* Amend list of contributors and sorted alphabetically by lastname
* Correct docstrings on some deprecated functions