bootstrap script accepts a version

[manics]

• [ ] Add / update documentation
• [x] Add tests

The first part of https://github.com/jupyterhub/the-littlest-jupyterhub/issues/724

This should allow:

• cat bootstrap.py | python3 - --version <branchname-or-dev-tag>
• cat bootstrap.py | python3 - --version <commit-sha>
• cat bootstrap.py | python3 - --version major.minor.patch
• cat bootstrap.py | python3 - --version major.minor latest major.minor release
• cat bootstrap.py | python3 - --version major latest major release
• cat bootstrap.py | python3 - --version latest latest released tag
• cat bootstrap.py | python3 - install from main branch (current default, though this should be changed to latest when there's a release tag)

Obviously this can't be properly tested since there are no tags :smiley:. However I've added some mock tests in test_bootstrap_functions.py. Since https://tljh.jupyter.org/en/latest/ is a live site we can't add documentation on using --version RELEASE until we've made a tag.