DS18B20 plugin prevents startup

If the crc module is not installed, mqtt4hass can't start, even if the DS18B20 plugin isn't used.

rtl@RTLSDR-Weather-MQTT-Bridge:~/mqtt4hass$ ./mqtt4hass.py
Traceback (most recent call last):
  File "/home/rtl/mqtt4hass/./mqtt4hass.py", line 458, in <module>
    sys.exit(main())
  File "/home/rtl/mqtt4hass/./mqtt4hass.py", line 433, in main
    ctrl = Controller()
  File "/home/rtl/mqtt4hass/./mqtt4hass.py", line 182, in __init__
    self.avialable_plugins = self.__discover_plugins()
  File "/home/rtl/mqtt4hass/./mqtt4hass.py", line 172, in __discover_plugins
    return {
  File "/home/rtl/mqtt4hass/./mqtt4hass.py", line 173, in <dictcomp>
    name: importlib.import_module(name)
  File "/usr/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/home/rtl/mqtt4hass/plugins/ds18b20/__init__.py", line 3, in <module>
    from crc import CrcCalculator, Crc8  # type: ignore
ModuleNotFoundError: No module named 'crc'

Either the crc module should be a global requirement (not ideal), or the import should be moved so it doesn't happen during plugin scan at startup.