v0.7.1.3

The same
libsecp256k1 0.7.1
(1a53f49) as v0.7.1.2: no breaking changes.

• A dynamic wheel's import failure says why. ImportError now names
  every shared-object candidate _load_lib rejected and chains the last
  loader error, instead of only "no loadable shared libsecp256k1 found".
• cryptography moved to 50.0.0, closing CVE-2026-69247 — a PKCS#7
  decryption oracle pulled in transitively through the packaging tools,
  and unreachable from this package's own code either way.
• Two README clarifications: why pubkey_from_prvkey is two
  libsecp256k1 calls rather than one, and which btclib functions guard a
  MuSig2 nonce against reuse.