Refactor dependencies for python versions

[nikhilym]

Description

This commit makes the following changes:

• Set 'typing' package to be installed only on Python 2.7. This fixes the import issue raised in Python 3.7, on using the 'typing 3.6' module, as mentioned in issue Callable has no attribute __abc_registry #49.
• Remove dependencies on enum34 and typing from extra_requires fields in setup files, since these are installed as dependencies in ask-sdk-model package.
• Set the install_requires dependencies to follow the same version as mentioned in requirements.txt in all ask-sdk* packages.

Motivation and Context

Fixes #49 , making the SDK compatible with Python 3.7

Testing

Ran tox and tested with couple of sample skills on Python 3.6, Python 3.7

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.