ecdsa 0.16.1

New API:

• VerifyingKey.precompute() supports lazy argument to delay precomputation
  to the first time the key is used to verify a signature.

Doc fixes:

• Documentation for the VerifyingKey.precompute() method.

Bug fix:

• Make created signatures correct when the hash used is bigger than the curve
  order bit size and the curve order is not a multiple of 8 (this affects
  only users of custom curves or hashes with output larger than 512 bits
  when used with NIST P-521 curve).

Performance:

• Speed up library load time by calculating the generator point multiplication
  tables the first time the points are used, not when they are initialised.

Maintenance:

• Include Python 3.9 in CI testing.
• Test coverage for the VerifyingKey.precompute() method.
• Small speed-ups for the test suite.