Fix 'contrib/update-copyright.py': 'TypeError: exceptions must derive from BaseException'

Running 'contrib/update-copyright.py' currently fails:

    [...]
    Traceback (most recent call last):
      File "contrib/update-copyright.py", line 365, in update_copyright
        canon_form = self.canonicalise_years (dir, filename, filter, years)
      File "contrib/update-copyright.py", line 270, in canonicalise_years
        (min_year, max_year) = self.year_range (years)
      File "contrib/update-copyright.py", line 253, in year_range
        year_list = [self.parse_year (year)
      File "contrib/update-copyright.py", line 253, in <listcomp>
        year_list = [self.parse_year (year)
      File "contrib/update-copyright.py", line 250, in parse_year
        raise self.BadYear (string)
    TypeError: exceptions must derive from BaseException

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "contrib/update-copyright.py", line 796, in <module>
        GCCCmdLine().main()
      File "contrib/update-copyright.py", line 527, in main
        self.copyright.process_tree (dir, filter)
      File "contrib/update-copyright.py", line 458, in process_tree
        self.process_file (dir, filename, filter)
      File "contrib/update-copyright.py", line 421, in process_file
        res = self.update_copyright (dir, filename, filter,
      File "contrib/update-copyright.py", line 366, in update_copyright
        except self.BadYear as e:
    TypeError: catching classes that do not inherit from BaseException is not allowed

Fix up for commit 3b25e835
"Port update-copyright.py to Python3".

	contrib/
	* update-copyright.py (class BadYear): Derive from 'Exception'.