Add support for Python 3.7+

[m-aciek]

• Uses typing syntax compatible with Python 3.7.
• Adapts the protocols definitions for earlier Python versions.
• Starts to test in CI Python versions from 3.7 to 3.12.
• Extends the test to support partial functionality for Python 3.7. I wasn't able to make NaiveDateTime/AwareDateTime work properly for Python 3.7 due to interface incompatibilities. Only Date/DateTime is supported as expected for Python 3.7. Full functionality for Python 3.8+.

Closes #6.