Djblets 3 drops support for Python 2.7 and 3.6. Python 2.7 reached

end-of-life, and 3.6 will reach end-of-life on December 23, 2021.
Python 3.7 is now the minimum supported version. It reaches end-of-life

on June 27, 2023.
Both 2.7 and 3.6 have been removed from the classifiers, dependency

ranges, and minimum versions. The old PYTHON_2_* constants have been

removed as well. This is not considered an API change, as the module

explicitly states that the contents within may change.
The package is no longer built as a universal wheel, and setup.py

won't run on older versions of Python.

Unit tests pass.
Built a package. Verified it was a Python 3-only wheel with the correct

metadata.