This copies the official Docker Python images approach of stripping
the Python test suite after building from source.

For Python 3.6.2 this reduces the output from 184MB to 121MB and
removes the following directories:
```
.heroku/python/lib/python3.6/tkinter/test
.heroku/python/lib/python3.6/ctypes/test
.heroku/python/lib/python3.6/unittest/test
.heroku/python/lib/python3.6/distutils/tests
.heroku/python/lib/python3.6/sqlite3/test
.heroku/python/lib/python3.6/lib2to3/tests
.heroku/python/lib/python3.6/test
```

For Python 2.7.13 this reduces the output from 127MB to 91MB and
removes the following directories:
```
.heroku/python/lib/python2.7/ctypes/test
.heroku/python/lib/python2.7/unittest/test
.heroku/python/lib/python2.7/distutils/tests
.heroku/python/lib/python2.7/bsddb/test
.heroku/python/lib/python2.7/lib-tk/test
.heroku/python/lib/python2.7/sqlite3/test
.heroku/python/lib/python2.7/lib2to3/tests
.heroku/python/lib/python2.7/json/tests
.heroku/python/lib/python2.7/email/test
.heroku/python/lib/python2.7/test
```

Fixes #424.