manager: python modules reorganization

Old knot_resolver module is now knot_resolver_build_options.

Old knot_resolver_manager module is now just knot_resolver.

- statistics.py: removed
- metrics: prometheus support is in separate module

- new KresManager exception
- imports formatting

Since we support python 3.8 and higher, there is no need for backwards support for most of the features we use in the standard typing library.