Add type annotations at cost of support of py2.7

[pyhedgehog]

Proposed changes

This PR adds type annotations to crossplane package. This closes #112.

Checklist

• [x] I have read the CONTRIBUTING doc
• [ ] I have added tests that prove my fix is effective or that my feature works (N/A)
• [x] I have checked that all unit tests pass after adding my changes
• [ ] I have updated necessary documentation (N/A)
• [x] I have rebased my branch onto master
• [x] I will ensure my PR is targeting the master branch and pulling from my branch from my own fork