[Dev/Test] Migrate PymatgenTest from unittest.TestCase to pytest

[DanielYang59]

Current pytest framework isn't fully compatible with unittest.TestCase, we should consider migrate to pytest framework:

The following pytest features do not work, and probably never will due to different design philosophies:

Fixtures (except for autouse fixtures, see below);
Parametrization;
Custom hooks;

---

For example pytest.mark.parametrize doesn't work:

• regression (but in the right direction?) with parametrize in unittest.TestCase pytest-dev/pytest#5382

from unittest import TestCase

import pytest


class TestPytest(TestCase):
    @pytest.mark.parametrize("foo", ["hello", "world"])
    def test_demo(self, foo):
        assert isinstance(foo, str)

Gives:

    def _callTestMethod(self, method):
>       if method() is not None:
E       TypeError: TestPytest.test_demo() missing 1 required positional argument: 'foo'

While the following works just fine:

- class TestPytest(TestCase):
+ class TestPytest: