Python-redux

- refactor(core): use `str_to_bool` of `python-strtobool` instead of `strtobool`
of `distutils`
- feat(test-snapshot): add prefix to snapshot fixture

- feat(test-snapshot): the `selector` function can signal the `monitor` it should
ignore a particular snapshot of the state by returning `None`

- refactor(test-snapshot): make it aligned with `pyfakefs` by using `try`/`except`
instead of checking `Path().exists()` as `pyfakefs` doesn't seem to respect `skip_names`
for `Path().exists()`

- feat(test-snapshot): while still taking snapshots of the whole state of the
store, one can narrow this down by providing a selector to the `snapshot` method
(used to be a property)
- feat(test-snapshot): new `monitor` method to let a test automatically take snapshots
of the store whenever it is changed. Takes an optional selector to narrow down
the snapshot.

- build(pypi): add metadata

- docs: add an introduction of `view` to `README.md`