Generate fat datafiles

pytz can still only read the deprecated 'fat' compiled timezone
datafiles.

Support for 'slim' datafiles should be done by using the
Python 3.9 parser and the available backports. Python 2.7
will be stuck only able to use the 'fat' format.

Makefile

@@ -138,7 +138,7 @@ upload_docs_pythonhosted: docs
 	touch $@
 
 .stamp-zoneinfo:
-	${MAKE} -C ${IANA} TOPDIR=`pwd`/build USRDIR= USRSHAREDIR=etc install
+	${MAKE} -C ${IANA} ZFLAGS='-b fat' TOPDIR=`pwd`/build USRDIR= USRSHAREDIR=etc install
 	# Break hard links, working around http://bugs.python.org/issue8876.
 	for d in zoneinfo zoneinfo-leaps zoneinfo-posix; do \
 	    rm -rf `pwd`/build/etc/$$d.tmp; \