Bump debhelper from old 11 to 12.

Fixes: lintian: package-uses-old-debhelper-compat-version
See-also: https://lintian.debian.org/tags/package-uses-old-debhelper-compat-version.html

debian/changelog

+python-pygerrit2 (2.0.4-3) UNRELEASED; urgency=medium
+
+  * Bump debhelper from old 11 to 12.
+
+ -- Debian Janitor <janitor@jelmer.uk>  Mon, 13 Apr 2020 22:50:08 +0000
+
 python-pygerrit2 (2.0.4-2) unstable; urgency=medium
 
   * Team upload.

debian/control

@@ -5,7 +5,7 @@ Maintainer: Debian Python Modules Team <python-modules-team@lists.alioth.debian.
 Uploaders:
  Filip Pytloun <filip@pytloun.cz>,
 Build-Depends:
- debhelper-compat (= 11),
+ debhelper-compat (= 12),
  dh-python,
  python3-all,
  python3-pbr (>= 0.8.2),