Remove tools/pygettext.py (issue 2550977).

One of the miscellaneous Python 3 issues is making the script
tools/pygettext.py compatible with Python 3 (its use of tokenize, in
particular; issue 2550977).

Nothing else in the tree actually uses this script or references it at
all.  In particular, its stated reason for existence is that xgettext
doesn't understand Python, which is not true now.  Roundup's own
locale/GNUmakefile uses xgettext, not tools/pygettext.py, to generate
roundup.pot

If someone wants pygettext.py, there's a copy distributed with Python
itself:
<https://github.com/python/cpython/blob/master/Tools/i18n/pygettext.py>.
So there's no need for Roundup to have an old copy.

Author: jsm28