#1227 fix crash when proposed base python has no version data

[gaborbernat]

Resolves #1227.

[todo]

(stephenfin): Raise an exception here in tox 4.0

tox/src/tox/config/__init__.py

Lines 574 to 579 in 407121b

	# TODO(stephenfin): Raise an exception here in tox 4.0
	warnings.warn(
	"conflicting basepython version (set {}, should be {}) for env '{}';"
	"resolve conflict or set ignore_basepython_conflict".format(
	proposed_version, implied_version, testenv_config.envname
	)

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This comment was generated by todo based on a TODO comment in 407121b in #1229. cc @gaborbernat.

[asottile]