pipreqs takes the newst version, even if not supported by current python

I run `pipreqs --print` on my package, where I have a `jsonschema` import.

`pipreqs` gave me the `jsonschema==4.2.1` requirement, even though it requires `python>=3.7` and I ran `pipreqs` from a `python3.6` environment

Start by reproducing `pipreqs --print` in a Python 3.6 environment with a `jsonschema` import, then trace the dependency-version selection used by the pipreqs command. Done means the generated requirement selects a jsonschema release compatible with the running Python version.

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