0.54.0b0 - Conditional skip and xfail

Adds when param to @skip and @xfail decorators, allowing you to only apply them when some boolean or Callable predicate holds. e.g.

@skip("Skipped on Windows", when=platform.system() == "Windows")
@test("_build_package_name constructs package name '{pkg}' from '{path}'")
def _(
    pkg=each("", "foo", "foo.bar"),
    path=each("foo.py", "foo/bar.py", "foo/bar/baz.py"),
):
    m = ModuleType(name="")
    m.__file__ = path
    assert _build_package_name(m) == pkg


@skip("Skipped on Unix", when=platform.system() != "Windows")
@test("_build_package_name constructs package name '{pkg}' from '{path}'")
def _(
    pkg=each("", "foo", "foo.bar"),
    path=each("foo.py", "foo\\bar.py", "foo\\bar\\baz.py"),
):
    m = ModuleType(name="")
    m.__file__ = path
    assert _build_package_name(m) == pkg

When run on a non-Windows system: