Log Message:
py-ecdsa: updated to 0.16.0

Release 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).