py-ecdsa: updated to 0.16.1

Release 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).

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.


(adam)