debian: remove python3-pymavlink from dependencies

pymavlink is not available as an apt package, only via pip.
Remove it from hard dependencies and add note in description
that users must install it via pip.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: tridge

debian/control

@@ -15,7 +15,6 @@ Homepage: https://github.com/ArduPilot/MAVProxy
 Package: python3-mavproxy
 Architecture: all
 Depends: ${python3:Depends}, ${misc:Depends},
-  python3-pymavlink,
   python3-serial,
   python3-numpy
 Provides: ${python3:Provides}
@@ -29,5 +28,7 @@ Description: A MAVLink protocol proxy and ground station. MAVProxy
  for use in the 2012 Outback Challenge, and includes a module for the
  CanberraUAV search and rescue system.
  .
+ Requires pymavlink which must be installed via: pip3 install pymavlink
+ .
  See https://ardupilot.org/mavproxy/index.html for more information on how
  to use MAVProxy.