Add the following methods:
* assertHasAttr() and assertNotHasAttr()
* assertIsSubclass() and assertNotIsSubclass()
* assertStartsWith() and assertNotStartsWith()
* assertEndsWith() and assertNotEndsWith()
Also improve error messages for assertIsInstance() and
assertNotIsInstance().
On Windows, `os.path.isabs()` now returns `False` when given a path that
starts with exactly one (back)slash. This is more compatible with other
functions in `os.path`, and with Microsoft's own documentation.
Also adjust `pathlib.PureWindowsPath.is_absolute()` to call
`ntpath.isabs()`, which corrects its handling of partial UNC/device paths
like `//foo`.
Co-authored-by: Jon Foster <jon@jon-foster.co.uk>
Hugo van Kemenade