[cleanup] Remove a check that can never be reached

[Pierre-Sassoulas]

The package metadata makes sure that the python version is at least
3.7. This package won't be installed for other python interpreters.

[coveralls]

Pull Request Test Coverage Report for Build 6689778272

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 85.482%

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Files with Coverage Reduction	New Missed Lines	%
pylint_django/checkers/migrations.py	1	90.7%
pylint_django/plugin.py	1	80.0%

Totals
Change from base Build 6685340151:	-0.2%
Covered Lines:	736
Relevant Lines:	861

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 6689781794

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 85.714%

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Totals
Change from base Build 6685340151:	0.05%
Covered Lines:	738
Relevant Lines:	861

---

💛 - Coveralls