python3: Support Python2 for lcm when being used by drake_visualizer

[EricCousineau-TRI]

For #8352
Towards #9614

Allows us to use Python2 builds of director when building other stuff with Python3.

Install for Python2+3, using this setup script:

$ bazel run @lcm//:install -- ~+/build/install2
...
-- Installing: /home/eacousineau/proj/tri/repo/branches/drake/py3/drake/build/install2/lib/python2.7/site-packages/lcm/_lcm.so
-- Up-to-date: /home/eacousineau/proj/tri/repo/branches/drake/py3/drake/build/install2/lib/python2.7/site-packages/lcm/__init__.py
...
$ bazel-py3 run @lcm//:install -- ~+/build/install3
...
-- Installing: /home/eacousineau/proj/tri/repo/branches/drake/py3/drake/build/install3/lib/python3.5/site-packages/lcm/_lcm.cpython-35m-x86_64-linux-gnu.so
-- Up-to-date: /home/eacousineau/proj/tri/repo/branches/drake/py3/drake/build/install3/lib/python3.5/site-packages/lcm/__init__.py
-- Installing: /home/eacousineau/proj/tri/repo/branches/drake/py3/drake/build/install3/lib/python2.7/site-packages/lcm/_lcm.so
-- Up-to-date: /home/eacousineau/proj/tri/repo/branches/drake/py3/drake/build/install3/lib/python2.7/site-packages/lcm/__init__.py
...

Admittedly, the py2_py3 macro may be a little too cute for now. If we think it's OK to just always compile LCM in Python2 + Python3, I could just manually expand those targets.

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This change is 

[EricCousineau-TRI]

+@jamiesnape for review when you have a chance, please.

Let me know if there's a better packaging solution than this.

Reviewable status: all discussions resolved, platform LGTM missing

[EricCousineau-TRI]

Need to fix ctypes.cdll shenanigans...

[EricCousineau-TRI]

Fixed ctypes.cdll issue, but CI is showing stale results, possibly due to GitHub outages.

Reviewable status: all discussions resolved, LGTM missing from assignee jamiesnape, platform LGTM missing

[EricCousineau-TRI]

Re-adding "do not merge"; forgot that some of the code has "DO NOT MERGE" bits to exemplify what config could look like.

[jwnimmer-tri]

Would it be crazy to ship our pre-built drake-visualizer images with a private lcm-python that was already compiled? I'm not sure that we care about liblcm runtime compatibility between drake-visualizer and other python-based code within drake. If the visualizer is the only thing that needs python2 always, it might be a nice win to remove that use case from Drake's build, instead of adding all of this complexity.

[jamiesnape]

Not crazy at all if we keep LCM reasonably in sync (keeping on a tag would be nice).

[jamiesnape]

I think we only really care about wire compatibility in this case anyway, so the definition of “reasonably in sync” is loose.

[EricCousineau-TRI]

Aye, that'd work. How do we want to formulate that objective? Should I make a separate issue, or should we just track it under #8352?

Will close this PR once we do that.

[jamiesnape]

Create a separate issue and assign it to me. I assume the Python sources for the lcmtypes are 2/3 compatible?

[EricCousineau-TRI]

Seems that way; shall do!

[EricCousineau-TRI]

Shall be superseded by the solution to #9764