Add support for Metrics endpoint

[skupriienko]

Links:

• Jira

Actions:

• Add the Metrics endpoint:

  • Add the analytics key to Config's __getitem__ and special cases
  • Add mailgun/handlers/metrics_handler.py with handle_metrics()

• Examples:

  • Add mailgun/examples/metrics_examples.py with post_analytics_metrics() and post_analytics_usage_metrics()

• Tests:

  • Add class MetricsTest to tests/tests.py

• CI:

  • Update pre-commit hooks to the latest versions
  • Remove duplications
  • Add pydocstyle hook
  • Add types-requests to mypy's additional_dependencies
  • Clean up

• Package Management:

  • Breaking changes: drop support for Python 3.9
    • set py310 as a minimum version in environment-dev.yaml, environment.yaml, pyproject.toml, .github/ISSUE_TEMPLATE/bug_report.yml, .github/workflows/commit_checks.yaml
    • Remove _version.py from tracking and add to .gitignore
    • Improve a conda recipe
    • Enable refurb in environment-dev.yaml
    • Use project.license and project.license-files in pyproject.toml and remove wheel build dependency because of relying on setuptools >=77.

• Linting & Formatting:

  • Fix type hints in mailgun/handlers/domains_handler.py and mailgun/handlers/ip_pools_handler.py

• Docs:

  • Add Get account metrics and Get account usage metrics sections with examples to README.md
  • Update dependency pinning in README.md