fix: Do not revert to old broken import for pip 20+.[0-2]

[michael-k]

• Issue: sam build fails for python3.7 functions with pip==19.3 installed aws-sam-cli#1461 (original issue); fix: change the pip runner string based on a pip version check #132 (PR fixing the issue)

Description of changes:

This is a followup for #132. The current fix for pip 19.3+ only works for all releases of pip with a minor version >= 3. It will fail again with pip 20.0.

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