New API:

• Keys that use explicit curve parameters can now be read and written.
  Reading of explicit curves can be disabled by using the
  valid_curve_encodings keyword argument in VerifyingKey.from_pem(),
  VerifyingKey.from_der(), SigningKey.from_pem(), and
  SigningKey.from_der().
• Keys can now be written with use of explicit curve parameters,
  use curve_parameters_encoding keyword argument of VerifyingKey.to_pem(),
  VerifyingKey.to_der(), SigningKey.to_pem(), or SigningKey.to_der() to
  specify the format. By default named_curve will be used, unless the
  curve doesn't have an associated OID (as will be the case for an unsupported
  curve), then explicit encoding will be used.
• Allow specifying acceptable point formats when loading public keys
  (this also fixes a minor bug where python-ecdsa would accept raw
  encoding for points in PKCS#8 files). Set of accepted encodings is controlled
  by valid_encodings keyword argument in
  ECDH.load_received_public_key_bytes(), VerifyingKey.from_string(),
  VerifyingKey.from_pem(), VerifyingKey.from_der().
• PointJacobi and Point now inherit from AbstractPoint that implements
  the methods for parsing points. That added from_bytes() and
  to_bytes() methods to both of them.
• Curve parameters can now be read and written to PEM and DER files. The
  Curve class supports new to_der(), from_der(), to_pem(), and
  from_pem() methods.

Doc fix:

• Describe in detail which methods can raise RSZeroError, and that
  SigningKey.sign_deterministic() won't raise it.

Bug fix:

• Correctly truncate hash values larger than the curve order (only impacted
  custom curves and the curves added in this release).
• Correctly handle curves for which the order is larger than the prime
  (only impacted custom curves and the secp160r1 curve added in this release).
• Fix the handling of == and != for Public_key, Private_key, Point,
  PointJacobi, VerifyingKey, and SigningKey so that it behaves
  consistently and in the expected way both in Python 2 and Python 3.
• Implement lock-less algorithm inside PointJacobi for keeping shared state
  so that when calculation is aborted with KeyboardInterrupt, the state doesn't
  become corrupted (this fixes the occasional breakage of ecdsa in interactive
  shells).

New features:

• The speed.py script now provides performance for signature verification
  without use of precomputation.
• New curves supported: secp112r1, secp112r2, secp128r1, secp160r1.
• Keys with explicit curve encoding are now supported.

Performance:

• Use 2-ary Non-Adjacent Form for the combined multiply-add. This speeds up
  single-shot verify (i.e. without precomputation) by about 4 to 5%.
• Use native Python 3.8 support for calculating multiplicative inverses.

Maintenace:

• Include Python 3.9 in PyPI keywords.
• More realistic branch coverage counting (ignore Python version-specific
  branches).
• Additional test coverage to many parts of the library.
• Migrate to Github Actions for Continuous Testing.