Detect python version in pipenv

[rarkins]

Read Pipfile and as per https://pipenv-fork.readthedocs.io/en/latest/basics.html:

Specify your target Python version in your Pipfile’s [requires] section. Ideally, you should only have one target Python version, as this is a deployment tool. python_version should be in the format X.Y and python_full_version should be in X.Y.Z format.

When the format is X.Y, we need to decide between:

1. Appending .0 to it, or
2. Shipping with a hardcoded default for each minor stream (e.g. 3.7 -> 3.7.16), or
3. Dynamically looking up all available versions

(1) risks being too old. (2) may be a pragmatic trade-off between speed and accuracy. (3) ensures we always get the latest, but is slower/more error prone.