Support Python 3.15

[xfenix]

Motivation

Python 3.15 is out, but import microbootstrap fails on it:

ImportError: cannot import name '_MISSING_TYPE' from 'dataclasses'

microbootstrap.helpers imported the private dataclasses._MISSING_TYPE, which 3.15 removed: dataclasses.MISSING is now a sentinel object.

Changes

• dataclass_to_dict_no_defaults compares fields with is dataclasses.MISSING. This is the public API and works on every supported version.
• pyroscope-io has no 3.15 wheels yet and needs a Rust toolchain to build from source. It is now skipped on 3.15, same as on Windows. The import guard already makes the Pyroscope instrument report "not ready" in this case. The marker can go once upstream ships 3.15 wheels.
• 3.15 is added to the CI matrix and the classifiers. README and CLAUDE.md are updated.

Testing

Locally with uv lock --upgrade, as just install runs in CI:

• 3.15: 343 passed, 1 skipped (Pyroscope tests skip because the package is not installed)
• 3.10: 345 passed, 1 skipped
• ruff format --check, ruff check, mypy pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	99.34% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
microbootstrap/helpers.py	98.33% <100.00%> (-0.03%)	⬇️
microbootstrap/instruments/pyroscope_instrument.py	100.00% <ø> (ø)

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