Please refer to wiki/UsingPython3.
Melodic will not be migrated, primarily as from its supported list of platforms only Ubuntu Bionic is still supported (and the Python 2 there will be supported by Canonical for as long as needed (see Some things to know as Python 2 approaches EOL on ROS Discourse)).
The first (and only) ROS 1 release to support Python 3 will be Noetic (see ros-infrastructure/rep#202).
Additional links:
Duplicates/related Q&As:
I'm trying to write some ROS code and future proof it, and I find I'm wasting a ton of time recompiling everything to support Python3. Just writing a simple 10 line script to build a bag file requires recompiling nearly every package in ROS.
depending on what you actually need there is a relatively low-overhead work-around: installing Python-only ROS packages in a Python 3 venv. See rospypi/simple:
python3 -m venv venv_rospy
source venv_rospy/bin/activate
pip install --extra-index-url https://rospypi.github.io/simple/ rospy
...
no need to compile anything.
This can be made as complex as you'd like, but the more custom you make it, the more time you'll "waste" on setting things up.
For simple pub/sub, service and action work this is a nice enough work-around until Noetic is here.
