tests: Upgrade coverage to 7.6.1

[adamchainz]

Fixes these warnings seen during tests on Python 3.12:

/.../coverage/pytracer.py:223: DeprecationWarning: currentThread() is deprecated, use current_thread() instead
  self.thread = self.threading.currentThread()
  
...

/.../coverage/parser.py:382: DeprecationWarning: co_lnotab is deprecated, use co_lines instead.
  if not hasattr(self.code, attr):

The last one is repeated many times.

Coverage 7.6.1 is compatible with Python 3.8-3.12, so perfect for the range tested here.

Submitting Pull Requests

General

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  Examples: "fix(google): Fixed foobar bug", "feat(accounts): Added foobar feature".
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• If your changes are significant, please update ChangeLog.rst.
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[coveralls]

coverage: 96.117% (+0.3%) from 95.804%
when pulling 6e8f3aa on adamchainz:upgrade_coverage
into 03debcf on pennersr:main.