Supermemo2

- 🐞 Update all review_date references to review_datetime, somewhere missed in the previous release.

All changes are from PR 16 by alankan886

Breaking changes
- ♻️ Refactor code and simplify usage for v3.0.0. Making the required python to 3.8+.

Docs
- 📚 Update README to match new version v3.0.0.

Tests
- ✔️ Update unit tests for new v3.0.0 code.

Internal
- 👷 Modify Github Action on checking on Python 3.8+ versions only.
- 👷 Change default branch name from `master` to `main`.

All changes are from PR 13 by alankan886 and blazej-czapp.

Breaking changes
- ♻️ Refactor code and simplify usage for v2.0.0.

Docs
- 📚 Update README to match new version v2.0.0.

Tests
- ✔️ Update unit tests for new v2.0.0 code.

Internal
- 👷 Modify Github Action on checking on all Python 3 versions.

Fixes
- 🐞 Re-evaluate the default date argument to first_review() on each call. PR 10 by blazej-czapp.

Fixes
- 🐞 Specify attrs version and update changelog. PR 6 by praneetheus.
- 🐞 Fix import issue on SMTwo model. PR 7 by alankan886.
- 🐞 Modify SMTwo E-Factor calculates when quality < 3. PR 9 by alankan886.

Docs
- 📚 Modify docs according to the bug fixes and add a note on misleading package title.

Tests
- ✔️ Change test to adopt the E-Factor calculation change.

Internal
- 🔥 Remove tox.ini and PYTHONPATH in ci.yml.

Fixes
* ✔️ Add missing assertions to test_api.py.

Docs
* 📚 Update README badges and fix format.

Internal
* 👷 Add Github actions to run tests against Python versions 3.6 to 3.9 in different OS, and upload coverage to Codecov.