Changed assertions and fixtures

[1rvine]

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Partially addresses #1228

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Replaced unittest assertions with Pytest assertions and replaced setup and teardown methods with python fixtures

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test_core.py & test_opentime.py

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[linux-foundation-easycla]

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[reinecke]

Looks like it's coming along!

In addition to the specific notes I gave, here are some more general notes to apply broadly across the PR:

• It looks like the build failed mostly due to linting errors. Check out PEP 8 and The Black code style to get familiar with python code style guidelines.
• Simple assertions (like assert (t.value == 0) or assert(tr.contains(tstart))) should not include the parens, they can just be written as assert t.value == 0 or assert tr.contains(tstart)
• Since the goal of the PR is to move from unittest to pytest, try removing the import unittest from the top of the files. What errors pop up when you run the tests? Those are all things that moving to pytest's standards will help address.

[reinecke]

Since this PR is aiming to switch from unittest to pytest, I'd look at the uses of unittest within this module and see if you can replace them with pytest equivalents.

One example I see is the usage of the @unittest.skipUnless decorator - this can be replaced by "inverting" the logic on the test case and using @pytest.mark.skipif instead.