Debian packaging fixes

The following patch enables Python2 and Python3 properly, and allows building for Debian Bullseye:

diff -Nur old/debian/control new/debian/control
--- old/debian/control  2022-08-03 04:42:49.000000000 +0200
+++ new/debian/control  2022-08-15 12:55:04.000000000 +0200
@@ -9,7 +9,8 @@
  doxygen,
  gawk,
  pkg-config,
- python,
+ python-dev,
+ python3-dev,
  libkrb5-dev,
  libsasl2-dev,
  libsasl2-modules,
@@ -17,6 +18,8 @@
  libboost-filesystem-dev
 Standards-Version: 3.9.8
 Homepage: https://wiki.ligo.org/Computing/NDSClient
+X-Python-Version: >= 2.7
+X-Python3-Version: >= 3.5
 
 Package: nds2-client
 Architecture: any
diff -Nur old/debian/rules new/debian/rules
--- old/debian/rules    2022-08-03 04:42:49.000000000 +0200
+++ new/debian/rules    2022-08-15 12:55:04.000000000 +0200
@@ -4,7 +4,7 @@
 
 
 %: debian/octave-nds2-client.dirs debian/octave-nds2-client.install
-       dh $@ --buildsystem=cmake --with python2
+       dh $@ --buildsystem=cmake --with python2,python3
 
 debian/octave-nds2-client.dirs:
        echo $(OCT_SITE_DIR) >$@