update CHANGELOG.rst & AUTHORS.rst (#3231)

* finish https://github.com/pyca/cryptography/pull/1973

* change API & add test

Function will now return an instance of EllipticCurvePrivateKey, as that
is the users' ultimate goal anyway.

* fix test

* improve coverage

* complete coverage

* final fix

* centos fix

* try ec.SECT283K1

* try ec.SECT571K1

* try ec.SECT409K1

* try ec.SECT283K1

* try ec.SECT233K1

* try ec.SECT163K1

* try ec.SECT571R1

* try ec.SECT409R1

* try ec.SECT283R1

* try ec.SECT233R1

* try ec.SECT163R2

* try ec.SECP521R1

* try ec.SECP256R1

* retry

* cleanup asserts

* use openssl_assert

* skip unsupported platforms

* change API name to derive_private_key

* change version added

* improve description of `secret` param

* separate successful and failure test cases

* simplify successful case

* add docs for derive_elliptic_curve_public_point

* add period

* update CHANGELOG.rst & AUTHORS.rst

for https://github.com/pyca/cryptography/pull/3225

* added reST prefix

* reduce line length
2 files changed