Drop Python 3.8 support

[khvn26]

Blocked on Flagsmith/flagsmith-engine#245.

• Remove 3.8 from testing matrix in CI
• Remove 3.8 from pyproject.toml

[khvn26]

Released as part of https://github.com/Flagsmith/flagsmith-python-client/releases/tag/v4.0.0.