Fix 3.8 and 3.9 wheels for linux x86_64

Actually use manylinux2014 docker image. (#194)

Actually use the manylinux2014 docker images, instead of just running on an EC2 instance that's similar enough that we happen to pass the wheel audit.

Also, setup.py improvements:
- accept `cmake3` as well as `cmake`.
- slight cleanup of how AWS_LIBCRYPTO_INSTALL is used.

Fix X-Amz-Security-Token bug

• BUGFIX: MQTT CONNACK with error code results in failed connection.
• BUGFIX: Fix X-Amz-Security-Token bug in HTTP request signing.
• BUGFIX: Fix ecc signature verification failures under certain cases on Windows.
• MINOR CHANGE: Accept duck typed python streams.

Updated to s2n v0.10.19 (#186)

Updated to s2n v0.10.19 (#186)

Increase default TLS timeout

• BUGFIX: Increased default TLS timeout. This was a frequent cause of connection failure.
• BUGFIX: Fix how SigV4 canonicalizes multiline header values.

Remove Python 2 support

• MAJOR CHANGE: Remove support for Python 2, Python 2.7 was end of life Jan 1, 2020.
• MAJOR CHANGE: Stop releasing wheels for Python 3.4, which was end of life in 2019.
• BUGFIX: Fixed places where unicode strings were not properly passed to C.
• INTERNAL: Many C objects now use ref-counting under the hood. This does not affect the python API.

iOS API adjustments

• Fixes a crash when shutting down an mqtt connection with an incomplete request with no callback from aws-c-mqtt
• Added check for iOS platform

Auth signing change

• API CHANGE: awscrt.auth.AwsSigningConfig.signed_body_value is now a string instead of an enum.
• BUGFIX: websocket no longer hangs if CLOSE frame cannot be sent.

MQTT bug fix

• Improved Mac-Only TLS log for private key in aws-c-io
• Solve the race condition from aws-c-mqtt

Grab bag of fixes via dependencies

• Ecs provider crash fix (auth)
• Tls window calculation fix (io)
• Http monitoring options unitialization fix (http)

Auth signing revamp

API changes to auth.py, in accordance with changes to the underlying aws-c-auth