Remove 2to3-done.txt (issue2550979).

With all the mechanical and most of the non-mechanical parts of Python
3 support done, the lists of source files in 2to3-done.txt are no
longer relevant.  The discussion of plans for Python 3 support is also
no longer relevant, with the string representations being described in
a comment in roundup/anypy/strings.py and other known outstanding
issues being described in open issues in the tracker.

Author: jsm28

2to3-done.txt

@@ -9,6 +9,5 @@ recursive-include detectors *.py README.txt
 global-exclude *.pyc *.pyo .DS_Store *.orig *.rej *~ \#*
 include run_tests.py demo.py *.txt
 include doc/conf.py doc/roundup-favicon.ico
-exclude 2to3-done.txt
 exclude doc/security.txt
 include locale/*.po locale/*.mo locale/roundup.pot