* Relase 0.17.0 (27 May 2021)

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.

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.