Update cryptography requirement from 2.5 to 2.6.1

[dependabot-preview]

Updates the requirements on cryptography to permit the latest version.

Changelog

Sourced from cryptography's changelog.

2.6.1 - 2019-02-27

• Resolved an error in our build infrastructure that broke our Python3 wheels for macOS and Linux.

2.6 - 2019-02-27

• BACKWARDS INCOMPATIBLE: Removed cryptography.hazmat.primitives.asymmetric.utils.encode_rfc6979_signature and cryptography.hazmat.primitives.asymmetric.utils.decode_rfc6979_signature, which had been deprecated for nearly 4 years. Use ~cryptography.hazmat.primitives.asymmetric.utils.encode_dss_signature and ~cryptography.hazmat.primitives.asymmetric.utils.decode_dss_signature instead.
• BACKWARDS INCOMPATIBLE: Removed cryptography.x509.Certificate.serial, which had been deprecated for nearly 3 years. Use ~cryptography.x509.Certificate.serial_number instead.
• Updated Windows, macOS, and manylinux1 wheels to be compiled with OpenSSL 1.1.1b.
• Added support for /hazmat/primitives/asymmetric/ed448 when using OpenSSL 1.1.1b or newer.
• Added support for /hazmat/primitives/asymmetric/ed25519 when using OpenSSL 1.1.1b or newer.
• ~cryptography.hazmat.primitives.serialization.load_ssh_public_key can now load ed25519 public keys.
• Add support for easily mapping an object identifier to its elliptic curve class via ~cryptography.hazmat.primitives.asymmetric.ec.get_curve_for_oid.
• Add support for OpenSSL when compiled with the no-engine (OPENSSL_NO_ENGINE) flag.

2.5 - 2019-01-22

• BACKWARDS INCOMPATIBLE: U-label strings were deprecated in version 2.1, but this version removes the default idna dependency as well. If you still need this deprecated path please install cryptography with the idna extra: pip install cryptography[idna].
• BACKWARDS INCOMPATIBLE: The minimum supported PyPy version is now 5.4.
• Numerous classes and functions have been updated to allow bytes-like types for keying material and passwords, including symmetric algorithms, AEAD ciphers, KDFs, loading asymmetric keys, and one time password classes.
• Updated Windows, macOS, and manylinux1 wheels to be compiled with OpenSSL 1.1.1a.
• Added support for ~cryptography.hazmat.primitives.hashes.SHA512_224 and ~cryptography.hazmat.primitives.hashes.SHA512_256 when using OpenSSL 1.1.1.
• Added support for ~cryptography.hazmat.primitives.hashes.SHA3_224, ~cryptography.hazmat.primitives.hashes.SHA3_256, ~cryptography.hazmat.primitives.hashes.SHA3_384, and ~cryptography.hazmat.primitives.hashes.SHA3_512 when using OpenSSL 1.1.1.
• Added support for /hazmat/primitives/asymmetric/x448 when using OpenSSL 1.1.1.
• Added support for ~cryptography.hazmat.primitives.hashes.SHAKE128 and ~cryptography.hazmat.primitives.hashes.SHAKE256 when using OpenSSL 1.1.1.
• Added initial support for parsing PKCS12 files with ~cryptography.hazmat.primitives.serialization.pkcs12.load_key_and_certificates.
• Added support for ~cryptography.x509.IssuingDistributionPoint.
• Added rfc4514_string() method to x509.Name <cryptography.x509.Name.rfc4514_string>, x509.RelativeDistinguishedName
  <cryptography.x509.RelativeDistinguishedName.rfc4514_string>, and x509.NameAttribute <cryptography.x509.NameAttribute.rfc4514_string> to format the name or component an 4514 Distinguished Name string.
• Added ~cryptography.hazmat.primitives.asymmetric.ec.EllipticCurvePublicKey.from_encoded_point, which immediately checks if the point is on the curve and supports compressed points. Deprecated the previous method ~cryptography.hazmat.primitives.asymmetric.ec.EllipticCurvePublicNumbers.from_encoded_point.
• Added ~cryptography.x509.ocsp.OCSPResponse.signature_hash_algorithm to OCSPResponse.
• Updated /hazmat/primitives/asymmetric/x25519 support to allow additional serialization methods. Calling ~cryptography.hazmat.primitives.asymmetric.x25519.X25519PublicKey.public_bytes with no arguments has been deprecated.
• Added support for encoding compressed and uncompressed points via ~cryptography.hazmat.primitives.asymmetric.ec.EllipticCurvePublicKey.public_bytes. Deprecated the previous method ~cryptography.hazmat.primitives.asymmetric.ec.EllipticCurvePublicNumbers.encode_point.

2.4.2 - 2018-11-21

• Updated Windows, macOS, and manylinux1 wheels to be compiled with OpenSSL 1.1.0j.

2.4.1 - 2018-11-11

• Fixed a build breakage in our manylinux1 wheels.

2.4 - 2018-11-11

... (truncated)

Commits

• e457053 2.6.1 release with fixed wheels (#4792)
• 659cffe Don't use pep517 when building our wheel (#4790)
• da25920 bump version and update changelog for 2.6 release (#4787)
• 1f4e646 support ed25519 openssh public keys (#4785)
• 871e97a ed448 support (#4610)
• 4c77bf3 homebrew switched from --build-bottle to --build-from-source (#4786)
• cd05e2a ed25519 support (#4114)
• 6a02279 remove unused x509 bindings (#4776)
• b44fc56 update travis builders to newer versions of openssl (#4784)
• 8b2f548 remove unused locking functions (#4780)
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #358.