Fix a flaky test by cleaning a polluted state

[sturmianseq]

What is the purpose of the change

This PR is to fix a flaky test test/test_occam_data.py::test_occam_data_not_save_to_npy, which can fail after running test/test_occam_data.py::test_occam_data_attributes, but passes when it is run in isolation.

Reproduce the test failure

Run the following command:

python -m pytest test/test_occam_data.py::test_occam_data_attributes test/test_occam_data.py::test_occam_data_not_save_to_npy

Expected result

test/test_occam_data.py::test_occam_data_not_save_to_npy should pass after running test/test_occam_data.py::test_occam_data_attributes.

Actual result

    def test_occam_data_not_save_to_npy():
>       assert not os.path.exists(class_dir)
E       AssertionError: assert not True
E        +  where True = <function exists at 0x7f3b6d24cca0>('/home/yyy/pythonOD/explore_test/cprojects/OccamTools/test/../data/class_data')
E        +    where <function exists at 0x7f3b6d24cca0> = <module 'posixpath' from '/usr/lib/python3.8/posixpath.py'>.exists
E        +      where <module 'posixpath' from '/usr/lib/python3.8/posixpath.py'> = os.path

test/test_occam_data.py:217: AssertionError

Why it fails

The test class directory is not deleted after running test/test_occam_data.py::test_occam_data_attributes, which leads to a pollution.

Fix

Delete class_dir at the end of test/test_occam_data.py::test_occam_data_attributes

[codecov]

Codecov Report

Merging #15 (fb07682) into master (fcaaac4) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #15   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         2328      2329    +1     
=========================================
+ Hits          2328      2329    +1     

Impacted Files	Coverage Δ
test/test_occam_data.py	100.00% <100.00%> (ø)

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