refactor: test suite modules, f-strings, setup requires fix

[szymonlopaciuk]

• refactor errors to use python 3 f-strings
• remove overlap between tests_require and install_requires in setup.py
• make test suites modules (by adding __init__.py to each) in order to allow the same file names in different suites

[coveralls]

Pull Request Test Coverage Report for Build 54

• 5 of 7 (71.43%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.388%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
inspire_mitmproxy/errors.py	5	7	71.43%

Totals
Change from base Build 53:	0.0%
Covered Lines:	191
Relevant Lines:	209

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💛 - Coveralls