Change deprecation mechanism to use a function

As discussed in https://github.com/pytest-dev/pytest/pull/7362#issuecomment-649800119_, let’s change our warnings mechanism to include a new PytestExpiredDeprecationWarning exception which is chosen at runtime depending on the current pytest version.

This will simplify our side because we won’t need to change PytestDeprecationWarnings into errors anymore before each major release.

[graingert]

Remember the stacklevel for warnings.warn:

def deprecated(reason: str, *, due: int, stacklevel=0):
    stacklevel = stacklevel + 1
    if due >= pytest version [0]:
        warnings.warn(PytestExpiredDeprecationWarning(reason), stacklevel=stacklevel)
    else:
        warnings.warn(PytestDeprecationWarning(reason), stacklevel=stacklevel)

[RonnyPfannschmidt]

based on experiences in pip and a few internal tools, the self version dependency seems orders of magnitude better than any manually managed way to deal with it