Packaging cleanup

* Declare compliance with policy 4.5.1
* Bump debhelper compat level to 13
* Update debian/watch to version 4

debian/control

@@ -1,11 +1,11 @@
 Source: python-magic
 Maintainer: Christoph Biedl <debian.axhn@manchmal.in-ulm.de>
 Homepage: https://github.com/ahupp/python-magic/
-Standards-Version: 4.4.1
+Standards-Version: 4.5.1
 Vcs-Browser: https://git.in-ulm.de/cbiedl/python-magic
 Vcs-Git: https://git.in-ulm.de/cbiedl/python-magic.git
 Testsuite: autopkgtest-pkg-python
-Build-Depends: debhelper-compat (= 12),
+Build-Depends: debhelper-compat (= 13),
     dh-python,
     libmagic1 (>= 1:5.39),
     python3-all,

debian/watch

@@ -1,4 +1,4 @@
 
-version=3
-https://github.com/ahupp/python-magic/releases \
-    /ahupp/python-magic/archive/([0-9][.0-9]*)\.tar\.gz
+version=4
+https://github.com/ahupp/@PACKAGE@/releases \
+    /ahupp/@PACKAGE@/archive/@ANY_VERSION@@ARCHIVE_EXT@