Python-mbedtls

Update version in `setup.py`. No change in the code.

* x509: Add accessors to most fields of CRT, CSR, and CRL.
The `x509` module can now be used to inspect existing certificates.
* x509: The module now offers basic signing and
verification capabilities.

API Changes

* pk: `from_{PEM,DER,buffer}` are now `classmethod`s to
comply with PEP 543.
* x509: `Certificate` class renamed `CRT`.
* x509: `start` and `end` arguments renamed `not_before` and
`not_after`; `md_alg` argument is now `digestmod`.
* x509: Re-order the arguments of the `new()` methods.
* exceptions: Rename `MbedTLSError` -> `TLSError` to comply
with PEP 543.

Misc

* pk: `__eq__()` now also accepts DER (bytes) or PEM (str) formats.
* x509: `__bytes__()` returns the certificates in the DER format.
* x509: `__str__()` now returns a human-readable string.

Update changelog. No changes in the code.

* Support Diffie-Hellman-Merkle key exchange.
* MPIs (multi-precision integers) now implement the full
`numbers.Integral` API.
* MPIs are erased from memory upon garbage collection.
* The `mpi` library is now public (renamed `_mpi` -> `mpi`).

API Changes

* pk: Methods that were previously returning a long integer now
return an MPI.
* exceptions: Rename `_ErrorBase` -> `MbedTLSError`. It is now
the only new exception.
* exceptions: `mbedtls_strerror()` generates the error message.

Support elliptic curve cryptography

* pk/ECC: Elliptic curve digital signature algorithm.
* pk/ECDHServer, pk/ECDHClient: Elliptic curve Diffie-Hellman.
The two classes together support ECDHE (ephemeral) as well.

API Changes

* pk/RSA: generate() returns the private key.
* pk/RSA: to_DER() and to_PEM() are obsolete and will be removed.
Users should call export_key() and export_public_key() instead.
* pk/RSA: has_public() and has_private() are now private.
Test if export_key() and export_public_key() are truthy
or falsy instead.
* pk/RSA: __str__() now returns the private key in PEM format.
* pk/RSA: to_bytes() and __bytes__() now return the private key
in DER format.

Bugfix

* pk: Increase the size of the buffer for sign, encrypt, decrypt

Misc.

* Use const memoryviews supported by Cython 0.28.0.
* pk: pk is now a module instead of a package.

API Changes

* x509/Certificate: next() returns the next certificate in a chain.
* md: Implement block_size property.

Misc.

* Clean up imports.
* Fix tests and packaging.
* Change git merge policy to fast-forward on develop.