Remove local install python in favour of hdev and fix tests

[jon-betts]

For: https://github.com/hypothesis/tox-pip-sync/issues/14 and https://github.com/hypothesis/hdev/issues/44

This removes the install python script and replaces it with the hdev install-python command. This also includes the fix for tox-pip-sync preventing testing updating.

Testing notes

I don't know if you're really going to want to do this for all these very similar PR's but here's how you would:

Install python script

• make format - Check that the python install works
• Change the first version number in .python-versions
• make format - You should see Python start installing
• If you wait for it to finish you can re-run make format and it should not be installed again

tox-pip-sync fix:

• rm .tox/.tox -rf - To ensure you have up to date tox-pip-sync
• Run make test
• Edit the main __init__.py in the src/<package_name> dir
• Add raise Exception("live!")
• Run make test
• The tests should fail - In main this would use the same code as the first step and work fine