Python-mbedtls

* cipher: CBC does not use PKCS7 padding by default and now requires a full
block. Encoding will raise ValueError otherwise.
* tls: Add support for session caching.
* tls: Implement `context.getpeercert()`.
* tls: Add context manager to `TLSWrappedSocket`
* programs: Add example DTLS and TLS client and server.
* ci: Drop CircleCI.
* Update wheels to mbedtls 2.16.12
* Add support for Python 3.10.
* Drop Python 3.6 (EOL).

* tls: Fix arguments of TLSWrappedSocket.sendto() method.
* *: Add MacOS support and tests on GitHub.
* *: Add Windows support and tests on GitHub.
* *: Remove Travis-CI job: The main CI is moved to GitHub
Actions because of the less FOSS-friendly policy by Travis CI.
* *: Implement the pickling protocol (PEP 307).
RingBuffer, ciphers, TLSConfiguration and DTLSConfiguration,
and x509 certificates are copyable. The other Cython classes
are non-copyable.
* tls: Fix missing DTLS-only parameters in DTLSConfiguration
`__repr__()` and `__eq__()`. Equality, especially, could
have returned True for two configuration differing only on
some parameters.
* tls: Fix handling of min and max handshake timeout
when set independently.
* tls: Fix exception raised when unwrapping a
`TLSWrappedSocket` before opening it.
* tls: Augment size of the accept buffer for DTLS (nicho2).
* Update wheels to mbedtls 2.16.11.

* doc: Do not document Python 2.7-3.5 as documented ;p

* ci: Drop Python < 3.6.
* hashlib: Drop hash alias to hashlib.
* cipher,tls,x509: Unsupported ciphers now consistently
raise NotImplementedError.
* upstream: The size of the IV in ECB mode is consistently 0.
* Update wheels to mbedtls 2.16.9.
* cython: Get rid of compilation warnings.

* *: The '*.pyx' files were missing from the archive
on pypi.

* tls: Add accessors to min/max timeout DTLS handshake.
* ci: Support Python 3.9 final.
* ci: update wheels to mbedtls 2.16.8.
* ci: Prepare experimental support for Python 3.10.