New upstream release 2.3

Fixes possible tag truncation security bug in AEAD API, see #1602752

2.3 - 2018-07-18

• SECURITY ISSUE: finalize_with_tag() allowed tag truncation by default which can allow tag forgery in some cases. The method now enforces the min_tag_length provided to the GCM constructor.
• Added support for Python 3.7.
• Added extract_timestamp() to get the authenticated timestamp of a Fernet token.
• Support for Python 2.7.x without hmac.compare_digest has been deprecated. We will require Python 2.7.7 or higher (or 2.7.6 on Ubuntu) in the next cryptography release.
• Fixed multiple issues preventing cryptography from compiling against LibreSSL 2.7.x.
• Added get_revoked_certificate_by_serial_number for quick serial number searches in CRLs.
• The RelativeDistinguishedName class now preserves the order of attributes. Duplicate attributes now raise an error instead of silently discarding duplicates.
• aes_key_unwrap() and aes_key_unwrap_with_padding() now raise InvalidUnwrap if the wrapped key is an invalid length, instead of ValueError.