Add Debian 'Replaces: colcon' field (#628)

Upstream Debian has decided to package colcon, and they created a
package simply called 'colcon' which provides only the /usr/bin/colcon
executable and some weak dependencies as a sort of replacement for
colcon-common-extensions. This conflicts with our python3-colcon-core
package during unpacking.

We could add 'Conflicts: colcon', but I think this is a great use case
for 'Replaces:' because the only file provided by 'colcon' is
/usr/bin/colcon, which we obviously provide here. This way, folks can
still successfully install 'colcon' and get the weak dependencies even
if our version of python-colcon-core is installed, and our copy of
/usr/bin/colcon will take precedence over the one provided by 'colcon'.

stdeb.cfg

@@ -3,5 +3,6 @@ No-Python2:
 Depends3: python3-distlib, python3-empy (<4), python3-packaging, python3-pytest, python3-setuptools, python3 (>= 3.8) | python3-importlib-metadata
 Recommends3: python3-pytest-cov
 Suggests3: python3-pytest-repeat, python3-pytest-rerunfailures
+Replaces3: colcon
 Suite: focal jammy noble bookworm trixie
 X-Python3-Version: >= 3.6