Fix false negative for bad-string-format-type when using a variable as input

[DudeNr33]

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Type of Changes

	Type
✓	🐛 Bug fix
	✨ New feature
	🔨 Refactoring
	📜 Docs

Description

First and easy part of #6163.

After I started to work on this I realized that making bad-string-format-type and bad-format-character work on new-style formatting and f-strings will require a change to pylint.checkers.utils.parse_format_method_string.
As this also leads to small adjustments in the callers of this function, I would rather do this in a separate PR, and then continue working on bas-string-format-type and bad-format-character.

[DudeNr33]

To make up for the times a message is forgotten in check_messages we had bad-string-format-type twice here. 😀

[coveralls]

Pull Request Test Coverage Report for Build 2097408196

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 94.525%

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Totals
Change from base Build 2096791472:	0.003%
Covered Lines:	15712
Relevant Lines:	16622

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💛 - Coveralls