[crossfire] Python 3 conversion for trunk server users

[Kevin Zheng]

Hi folks,

With Python 3's end-of-life nearly upon us, it is time to make the great 
leap and switch to Python 3. Thanks to the work of many others, the 
switch now is relatively small.

Those who are running servers tracking trunk: after r21626, you should 
run cfdb_convert on existing databases. Some additional details about 
cfdb_convert are in the commit message, copied here:

  This Python 3 utility converts Python 2 bsddb 'shelve' databases to the
  default Python 3 'shelve' format.

  Server operators running existing servers should convert existing
  databases after switching to Python 3. Running cfdb_convert with the
  '--auto' flag in the 'var/crossfire' directory will automatically
  convert a list of standard databases with known names.

We anticipate minor fall-out from existing scripts that still use Python 
2 syntax, or result in run-time errors due to semantic changes in Python 
3. Please report them here, or on the list, so that they can be fixed as 
quickly as possible.

Regards,
Kevin