Proposed changes

• Fix bls.PopProve and bls.PopVerify to use the standard domain separation tag for proof-of-poessession purpose.
• According to https://www.ietf.org/archive/id/draft-irtf-cfrg-bls-signature-05.html#section-4.2.3,

  use BLS_SIG_BLS12381G2_XMD:SHA-256_SSWU_RO_POP_ for message signing and
  use BLS_POP_BLS12381G2_XMD:SHA-256_SSWU_RO_POP_ for proof-of-possession
  

• Added a test case from https://github.com/Chia-Network/bls-signatures/blob/2.0.2/src/test.cpp,
  which is the underlying library of python blspy and node.js bls-signatures.

Types of changes

• Bugfix
• New feature or enhancement
• Others

Checklist

• I have read the CONTRIBUTING GUIDELINES doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Related issues

Problem spotted by @ian0371 at #2043 (comment).

Further comments