Switch to using python3

[clalancette]

We are currently using python2 both for the launchers and for the python bindings. We eventually want to move to using python3, but this is blocked on the fact that LCM as compiled by drake only compiles for python2.7. Once we remove all of the bridge from delphyne, we don't need LCM anymore and we can then switch to python3.

[basicNew]

@EricCousineau-TRI qq: do you know if the pybind11 fork works with python 3? Any comments / warnings are more than welcome.

[stonier]

@EricCousineau-TRI, also interested in this question for TRI driving, since it defaults to python3.

[EricCousineau-TRI]

Our pybind11 fork is tested for Python3 on Ubuntu for GCC and Clang (and Mac, but not Windows, but that seems irrelevant at this point).
An example TravisCI build, from RobotLocomotion/pybind11#11:
https://travis-ci.org/RobotLocomotion/pybind11/builds/345471530

Once we remove all of the bridge from delphyne, we don't need LCM anymore and we can then switch to python3.

At present, Drake is only built for Python2.7, so if you're using pydrake, then there will be more blocking items. Will you be using pydrake in your fork, or re-binding classes?
If you wish to still use pydrake, then we should file an upstream issue; it may be relevant to smoothing over some of the silliness that's happening with Mac + Homebrew.

[stonier]

Unfortunately we're all far too dour and serious. No Mac silliness here ....

[stonier]

Python 2.7 for us is fine, for now. I'll let the TRI driving sim team entertain themselves with python3 / maliput bindings / pydrake and identify the issues for us.

[EricCousineau-TRI]

Sounds good.

I've realized that I haven't seen any issues regarding Python3 in Drake issues, so I made a quick one here:
RobotLocomotion/drake#8352

[stonier]

Subsumed by #544.

[clalancette]

We'd like to get this into Milestone 3, now that the Drake side is working.

[stonier]

Drake documentation re python3: https://drake.mit.edu/bazel.html#python-versions

Not sure if they have a fetchable tarball for a python3 build yet though. We'll need to either fetch that or build it and save it ourselves.

[EricCousineau-TRI]

Not yet, but should have one next week:
RobotLocomotion/drake#10408 (comment)

Also, CMake building:
https://drake.mit.edu/python_bindings.html#building-the-python-bindings

[basicNew]

Thanks for the pointer @EricCousineau-TRI ! Just to confirm, bazel run --config=python3 //:install <target_path> should to the trick for the installation part, right?

[EricCousineau-TRI]

Yup! You'll just need to make sure you run:

./setup/ubuntu/source_distribution/install_prereqs_user_environment.sh

first so it can symlink in the right platform. Note that Python 3 is only supported on Bionic.

[basicNew]

Ah, thanks for that @EricCousineau-TRI !

[clalancette]

This was completed by @basicNew and @hidmic in #583 and maliput/delphyne_gui#179 . @stonier would you like to pull the latest and check so we can close this issue (and thus close out Milestone 3)?

[basicNew]

Done, closing this one.