Add pylint -E and pytest to ci

[sezanzeb]

• Remove EOL python 3.7 from the ci
• Add pylint -E, fix some pylint errors
• Add pytest step
• Fix test_abs_values
• Turned RuntimeError into the desired UInputError if the device is not a character device, caused by a re-raise outside an except block
• Add test for S_ISCHR False

[sezanzeb]

TODO:

• Add a pylint -E step to the ci, check. Somehow make it work with the dynamic evdev/ecodes module.
• Figure out if evdev tests can work at all in a ci, or write some patches
• run input-remapper with those changes locally and check for errors