chore: release 3.1.0

[vvillait88]

Summary

Releases 3.1.0, carrying everything merged since 3.0.0:

• the quota denial message no longer tells an agent to retry, at all six adapters (fix: quota denial message no longer tells the agent to retry #153)
• to_security_requirements exported from identity and the package root (Export to_security_requirements from identity and the package root #151), which is the new public surface that makes this a minor
• A2A signing docs and supported versions corrected, README notes on UCP keys[] (README: UCP profile keys[] is informational #150, docs: correct A2A signing docs and supported versions; drop em-dashes #152)
• fastapi 0.142 resolves under the if-necessary prerelease rule with the web3<8 constraint (uv: let fastapi 0.142 resolve (prerelease if-necessary, web3<8) #149)

Also in this PR: agentscore-py floor raised to 2.7.1 (released today), and the web3<8 comment now states the real blocker. web3 8.0.0 is stable, but it needs eth-abi 6, which pympp's tempo extra excludes (eth-abi<6, pympp 0.11.0 is the latest), so an unconstrained re-lock lands on the 8.0.0b3 beta and pinning 8.0.0 downgrades pympp to 0.9.1. I tried the lift: the beta resolution passed the suite, but shipping a beta to merchants is the thing the constraint exists to prevent.

Worked with: Varun.

Out of scope: lifting web3<8, which waits on pympp admitting eth-abi 6.

Type of change

• Bug fix (no breaking change)
• New feature (no breaking change)
• Breaking change (existing callers must update)
• Docs, tests, or internal maintenance only

Public API

to_security_requirements is newly exported (merged in #151). Nothing removed or changed.

Test plan

ruff check, ruff format --check, ty check and pytest (1893 passed, 4 skipped, 95.41% coverage) pass locally on this branch.

Checklist

• Tests cover the new behavior, and the suite passes locally
• Lint, format, and type checks pass
• Docs and README examples updated if the public surface changed
• No secrets, credentials, or personal data in the diff or the tests