Update coverage requirement from <7 to <8

Updates the requirements on [coverage](https://github.com/nedbat/coveragepy) to permit the latest version.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](coveragepy/coveragepy@coverage-3.0b1...7.0.0)

---
updated-dependencies:
- dependency-name: coverage
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

setup.py

@@ -34,7 +34,7 @@ def _get_long_description():
     ],
     extras_require={
         'docs': ['sphinx >=3.0.0, <6', 'matplotlib >=3.1', 'numpydoc', 'm2r2'],
-        'tests': ['pytest', 'coverage <7', 'pytest-cov', 'coveralls'],
+        'tests': ['pytest', 'coverage <8', 'pytest-cov', 'coveralls'],
     },
     package_data={"csaps": ["py.typed"]},
     url='https://github.com/espdev/csaps',