Check-systemd

<small>[Compare with v0.4.0](https://github.com/Josef-Friedrich/check_systemd/compare/v4.0.0...v4.1.0)</small>

Added

- Add basic debug support (--debug) ([f371f78](https://github.com/Josef-Friedrich/check_systemd/commit/f371f78256bd0a407156abf7a507a3bdd4fcf328) by Josef Friedrich).

Fixed

- Fix check response for stopped units and units selected with --unit ([dd79ced](https://github.com/Josef-Friedrich/check_systemd/commit/dd79ceddcb23b7924b52c38e916aeea40a2d058e) by Josef Friedrich).

<small>[Compare with v3.1.0](https://github.com/Josef-Friedrich/check_systemd/compare/v3.1.0...v4.0.0)</small>

Changed

- Switch the testing framework to pytest ([a0aa63c](https://github.com/Josef-Friedrich/check_systemd/commit/a0aa63c0ff0aad038d9cc7f484cd7eee50744db5) by Josef Friedrich)

Fixed

- Fix the readthedocs site ([a7cf474](https://github.com/Josef-Friedrich/check_systemd/commit/a7cf4741500a981773d48d5f065ee5379828462e) by Josef Friedrich).
- Fix some type hint issues in the tests ([1e2b135](https://github.com/Josef-Friedrich/check_systemd/commit/1e2b135e92f919135eb7d937320d7878822ba99f) by Josef Friedrich).

Removed

- Remove the perfdata data_source ([62fd828](https://github.com/Josef-Friedrich/check_systemd/commit/62fd828177c9372a5234314e354ca9a442e726bb) by Josef Friedrich).

Added

- (Re)add the options `-i`, `--ignore-inactive-state` (31).

Added

- The options `--include`, `--include-unit`, `--include-type`,
`--exclude`, `--exclude-unit`, `--exclude-type` have been added to
have better control over which units should be selected for testing.
- A new entry was added to the performance data: `data_source=cli` or
`data_source=dbus`
- Add example `icinga2` configuration for the plugin (23)
- New option to query user units (`--user`) (22)
- New option `--required` to set the state that the systemd unit must have (17)

Changed

- The options `--dead-timers`, `--dead-timers-warning` and
`--dead-timers-critical` have been renamed to `--timers`,
`--timers-warning` and `--timers-critical`.
- In the command line help, the options have been grouped according to
their monitoring scope.
- Always return perfdata for `startup_time` even with `-n` (27)
- Exit `Unknown` when importing `nagiosplugin` fails (24)

Removed

- The options `-i`, `--ignore-inactive-state` have been removed.
- Drop support for 3.6 and 3.7

Fixed

- Clean up the documentation

Removed

- Remove the systemd source to avoid confusion (13)

Added

- Add option `--ignore-inactive-state` to prevent `check_systemd` from triggering a critical state