Switch "pypy3" keyword to point to 3.6 for backward compatibility

[maxim-lobanov]

Integrating changes from #164 to v1 version. Should be merged when #164 is tested and validated

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Currently, PyPy 2.7 and 3.6 are cached on Hosted images and 3.x selector works great.
In future, we are planning to add PyPy 3.7 too. In current implementation, selector 3.x will consume 3.7 and all customers will be switched automatically to 3.7 instead of 3.6 that could cause breaking changes.

We would like to make selector more specific to choose exact 3.6 to make sure that adding 3.7 to images won't break customers.
We are going to implement PyPy 3.7 support soon so customers will be able to select 3.6 and 3.7 separately.

Note:
Please pay attention that after this PR, core.setOutput('python-version', impl); will be set to pypy3.6 instead of pypy3. Not sure about impact from this change