use entry_points instead of a pre-written script

Summary:
This was the recommendation of
https://packaging.python.org/en/latest/distributing.html, but I unwisely
ignored it back when I wrote our setup.py. It turns out that getting a
launcher script to work properly on Windows is tricky. The
`entry_points` field generates a platform-appropriate launcher script at
install time, so we get Windows for free.

Previously our launcher script checked the version of Python and printed
a helpful error if it was too low. Because our main.py has `yield from`
syntax in it, printing such an error message isn't possible; we'll crash
with a syntax error first. We could create another file within the
modile (or maybe an __init__.py) to support this, but for now I'm just
dropping the feature.

Test Plan:
Manually played with peru.sh to make sure it still works. Ran
`pip install .` to check that the generated script works. Also tested
that on my Windows VM, and it works great. (It generates an exe file, in
fact.)

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D135
new test