fix: Deal with port as given by an env var, which might be a string.

[coveralls]

Pull Request Test Coverage Report for Build 4326383258

• 3 of 4 (75.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 90.941%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/pytest_mock_resources/config.py	2	3	66.67%
<!--	Total:	3	4	75.0%	-->

Totals
Change from base Build 4266917488:	-0.03%
Covered Lines:	1283
Relevant Lines:	1404

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💛 - Coveralls