Python-mbedtls

* setup.py: Refuse to build if mbedtls is too old.

* cipher: Increase test coverage and fix minor bugs.
* tests: Do not assume that havege is missing as it could be included
in some configurations.
* tests: Make error reporting with memoryviews closer to pytest
reporting for bytes.

* update wheels with 2.16.3.
* version: Make `has_feature()` public.
* tests: Skip tests to features that are not compiled in.
* tests: Improve error reporting with memoryviews.

* secrets: Add PEP-506 conform secrets module for generating secrets.
* cipher/hash/hmac: These modules and packages are now pure-Python.
* mpi: Add bitwise operations.

API Changes

* pk.ECDHNaive: `generate()` returns the public key as an ECPoint and
`import_peers_public()` now takes an ECPoint. `import_peer_public(MPI)`
is deprecated and will be removed in a future release. Further note
the `s` in `import_peers_public()`.
* pk: Make `ECDH*.{public_key,private_key,peers_public_key}` public. The
private equivalents with a `_*` prefix are deprecated and will be removed in a
future release. Further note the `s` in `peers_public_key`.
* _random: Privatize `Entropy` and refactor `Random` class.

Bugfixes

* mpi: Fix bug in `mpi.to_bytes()`.

Misc.

* *: Format Python files with `black`
* ci: Check that the docs build in CI.
* ci: Repair test coverage measurements on coveralls.
* mpi: Simplify code for comparisons.
* mpi: Add tests and better exceptions for `__pow__()`.

This is a bugfix release fixing several TLS issues.

Misc.

* Split download and install scripts.

Bugfixes

* tls: Split input and output buffers for TLS/TCP.
* tls: Fixes issue with TLSv1 by reading greedily from the buffer.

* Add support to HKDF (HMAC key derivation function).

Misc.

* Change branching model to fast-forward merge on the master branch.
* Change directive `language_level` from 2 to 3 for Cython.

Bugfixes

* *: Fixup handling of empty and `None` buffers (20: Roman (inpos)).
* hmac: Hmac now accepts the empty key (b"").