Adjust Python packages installed on Debian

The python-dnspython package was dropped from Debian "sid" and thus can
no longer be installed on that system.  Since this move is a part of a
larger initiative to remove Python 2 from Debian, there is little sense
in trying to implement Dockerfile workarounds for this specific package.
Instead, remove Python 2 packages from all our Debian Docker images
except the "buster" one (to retain some Python 2 test coverage for BIND
branches other than "master").

docker/bind9/debian-template/Dockerfile

@@ -66,9 +66,6 @@ RUN apt-get -y install		\
 	perl			\
 	pkg-config		\
 	procps			\
-	python			\
-	python-ply		\
-	python-dnspython	\
 	python3			\
 	python3-ply		\
 	python3-dnspython	\
@@ -78,6 +75,11 @@ RUN apt-get -y install		\
 	xsltproc		\
 	zip			\
 	zlib1g-dev
+RUN if [ "@VERSION@" = "buster" ]; then apt-get -y install \
+	python \
+	python-dnspython \
+	python-ply \
+; fi
 RUN if [ "@ABI_CHECK@" = "yes" ]; then \
 	apt-get -y install abi-compliance-checker abi-dumper w3m; \
 	# FIXME: This is Debian bug https://bugs.debian.org/952596. Before it