init command should warn about conflicted python versions

[T-256]

project init

rye init proj --py 3.7.9 --no-readme

pyptoject expected:

requires-python = ">= 3.7"

pyproject actual:

requires-python = ">= 3.8"

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I'm aware that we can config that by config.toml or --min-py cli option, but default version behavior when providing toolchain on init, could have improvement.

This also would prevent version conflicts like #82

[T-256]

possible fix from #82 (comment):

> rye init proj --py 3.7.9 --no-readme
new toolchain has conflict with project's required python ">=3.8" - auto fix it? (y/n) > y

pyproject.toml after auto fix:

requires-python = ">= 3.7"