Test on all OSs

[Swatinem]

This adds Windows and macOS to our CI test matrix.

It also changes quite a lot of internal functionality and tests to use Path.as_posix() which always prints paths with forward slashes.
This was primarily done to make a ton of tests work on Windows, but IMO it is also a good idea to normalize paths internally to use forward slashes.

---

As #661 is primarily an issue on Windows, it would be nice to run tests on Windows as well.
And also macOS as well, why not. This should cover the major OSs that folks will be running in their own CI jobs that are using the CLI.

[github-actions]

✅ All tests successful. No failed tests were found.

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[codecov]

❌ 5 Tests Failed:

Tests completed	Failed	Passed	Skipped
3585	5	3580	0

View the top 3 failed test(s) by shortest run time

api.temp.calculator.test_calculator::test_divide

Stack Traces | 0.001s run time

def
                test_divide():
                > assert Calculator.divide(1, 2) == 0.5
                E assert 1.0 == 0.5
                E + where 1.0 = <function Calculator.divide at 0x104c9eb90>(1, 2)
                E + where <function Calculator.divide at 0x104c9eb90> = Calculator.divide
                .../temp/calculator/test_calculator.py:30: AssertionError

api.temp.calculator.test_calculator::test_divide

Stack Traces | 0.001s run time

def
                test_divide():
                > assert Calculator.divide(1, 2) == 0.5
                E assert 1.0 == 0.5
                E + where 1.0 = <function Calculator.divide at 0x104c9eb90>(1, 2)
                E + where <function Calculator.divide at 0x104c9eb90> = Calculator.divide
                .../temp/calculator/test_calculator.py:30: AssertionError

api.temp.calculator.test_calculator::test_divide

Stack Traces | 0.001s run time

def
                test_divide():
                > assert Calculator.divide(1, 2) == 0.5
                E assert 1.0 == 0.5
                E + where 1.0 = <function Calculator.divide at 0x104c9eb90>(1, 2)
                E + where <function Calculator.divide at 0x104c9eb90> = Calculator.divide
                .../temp/calculator/test_calculator.py:30: AssertionError

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[joseph-sentry]

i think for the changes in folder_searcher to work with file finding you would need to make sure that the multipart_include_regex is also using as_posix(), this happens here:

codecov-cli/codecov_cli/services/upload/file_finder.py

Line 264 in c602edd

multipart_include_regex = globs_to_regex(

[joseph-sentry]

i think there's some required checks settings in the repo which is causing the CI to expect certain checks with diff names @thomasrockhu-codecov do you have perms to change those?

[thomasrockhu-codecov]

@joseph-sentry done