[formrecognizer] Remove 3.5 support

[kristapratico]

• We should GA version 3.1.0 with 3.5 support but include a message that it will not be supported moving forward:

This version will be the last version to officially support Python 3.5, future versions will require Python 2.7 or Python 3.6+

• Any version after 3.1.0 (GA or preview) will not support 3.5.

Please keep in mind that dropping support for a python version should indeed be a minor bump (not just patch).

[kristapratico]

Issue moved to Azure/azure-sdk-for-python-pr #768 via ZenHub