[BUG] Test coverage configuration duplicated

[dstansby]

Describe the bug
Not really a major bug, but the confiuration for collecting test coverage with pytest is duplicated in two places:

To Reproduce

python-cookiecutter/{{cookiecutter.package_name}}/pyproject.toml

Line 84 in 6fe2e39

addopts = "--cov={{cookiecutter.module_name}}"

python-cookiecutter/{{cookiecutter.package_name}}/tox.ini

Line 15 in 6fe2e39

pytest -v --color=yes --cov={{cookiecutter.module_name}} --cov-report=xml

Expected behaviour
Configuration only in one place. I'd suggest keeping it in pyproject.toml, as then the coverage configuration is accesible outside of using tox, but will still be used by tox.

Log file
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Screenshots
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Computer used (please complete the following information):

• OS: [e.g. Linux, Windows, macOS] For macOS, please specify if you have an Intel or M1 CPU (M1 is likely if your mac is new).
• Version [e.g. 18.04]
• Hardware specs [e.g. 128 GB RAM, Xeon 4114 CPU].

Additional context
Add any other context about the problem here.