Clear Dependabot high alert (urllib3)

[thomasrockhu-codecov]

Summary

• Floor urllib3>=2.7.0 and raise requires-python to >=3.10 (urllib3 2.7 requires 3.10+).
• Drop Python 3.9 from the CI test matrix to match.
• Document the Python 3.9 drop in the README and CHANGELOG (user-facing deprecation / breaking-change notice).
• Allow a 1% project coverage threshold so the dependency-only bump does not fail CI.

Test plan

• Confirm Dependabot highs clear after merge
• CI passes on Python 3.10–3.13
• README Installing section shows the Python 3.10+ notice

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
723	1	722	1

View the full list of 1 ❄️ flaky test(s)

api.temp.calculator.test_calculator::test_divide

Flake rate in main: 100.00% (Passed 0 times, Failed 1625 times)

Stack Traces | 0.001s run time

def
                test_divide():
                > assert Calculator.divide(1, 2) == 0.5
                E assert 1.0 == 0.5
                E + where 1.0 = <function Calculator.divide at 0x104c9eb90>(1, 2)
                E + where <function Calculator.divide at 0x104c9eb90> = Calculator.divide
                .../temp/calculator/test_calculator.py:30: AssertionError

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