ecdsa 0.16.0

New features:

• Support for reading and writing private keys in PKCS#8 format.

New API:

• to_pem and to_der now accept new parameter, format, to specify
  the format of the encoded files, either the dafault, legacy "ssleay", or
  the new pkcs8 to use PKCS#8. Note that only unencrypted PKCS#8 files are
  supported.
• Add allow_truncate to verify in VerifyingKey, it defaults to True,
  when specified as False, use of large hashes smaller than curves will be
  disallowed (as it was in 0.14.1 and earlier).

Bug fix:

• Correctly calculate signatures for private keys equal to n-1.
• Make PointJacobi and thus SigningKey and VerifyingKey pickleable.

Doc fixes:

• to_pem functions return bytes not str, document them as such.
• from_pem and from_pem in SigningKey returns SigningKey, document them
  as such.

Maintenance:

• Ensure that version checks will work with Python 4.
• Format the source with black.
• Fix uses of assert_ in test suite.
• Use newer Ubuntu in Travis to test against OpenSSL 1.1.1 (and thus
  test the interoperability of ECDH code in Travis).