lacking python3 support

[gijzelaerr]

Thinking astronomy is python2 only is really something of the past by now.

File "/usr/lib/python3.6/dist-packages/gsm/utils.py", line 156                                                                                                                                              
    except DBError, e:                                                                                                                                                                                        
                  ^                                                                                                                                                                                           
SyntaxError: invalid syntax