Python since 3.4 has included mock in the standard library, under the
unittest module. Since the lowest version supported is greater than
that, we can switch to using it and drop one external requirement.

* install via conda-forge

Co-authored-by: Lukas Kämmerling <github@lukas-kaemmerling.de>

This includes the remove of the upper boundary.

Signed-off-by: Lukas Kämmerling <lukas.kaemmerling@hetzner-cloud.de>

* Add validation helper for Label Values/Keys

This implementation is similar to the implementation in the hcloud-go. To give the "user" as much as verbosity as possible i introduced two methods.

- `validate` which is a simple validator that just returns True/False.add-validator-for-labels
- `validate_verbose` which also returns the corresponding error message (like which key/value is not correctly formatted)

I decided against calling one of both function in the other and instead duplicate the code. This is based on my feeling that the code would be more complex when we as a sample call validate_verbose in validate. Also, it is slightly more readable imho, but we can discuss this :)

Closes #153

Signed-off-by: Lukas Kämmerling <lukas.kaemmerling@hetzner-cloud.de>

* Fix tox

Signed-off-by: Lukas Kämmerling <lukas.kaemmerling@hetzner-cloud.de>

Signed-off-by: Lukas Kämmerling <lukas.kaemmerling@hetzner-cloud.de>