When a package was removed from AUR, the check code didn't update the last_check
value and the loop code was always triggered.

[DEBUG] Next check is planned in -778781s, waiting for 1s
...
[DEBUG] Next check is planned in -778781s, waiting for 1s
...
[DEBUG] Next check is planned in -778781s, waiting for 1s
...
[DEBUG] Next check is planned in -778781s, waiting for 1s

Fix return NoneType in check:
[INFO] imapsync: Next check is planned in 3358s
[CRITICAL] '<' not supported between instances of 'int' and 'NoneType'
Traceback (most recent call last):
  File "/usr/bin/aurbot", line 458, in <module>
    Robot().start()
  File "/usr/bin/aurbot", line 449, in start
    timeout = min(next_checks)
TypeError: '<' not supported between instances of 'int' and 'NoneType'

- Put deamon code in Robot class
- Put package check/update code in Package class
- Use python3 f'string
- Use sentences for comments and docstrings

aurbot[3547325]: [CRITICAL] name 'fp' is not defined
aurbot[3547325]: [ERROR] Unknown error. Please report it with --debug.

Pacman hooks now handle these commands.

- Move maintainer check in update. So, no more maintainer check if there is
no need to build the update
- Move build code in a subfunction
- Improve some messages

Prevent later failure with invalid path in tarball

Since python 3.5 we need to raise an exception to prevent python to EINTR
see https://www.python.org/dev/peps/pep-0475/

Drop python-xdg deps