Flake8 maintenance for test suite continued

[dcslagel]

Description:

This change covers flake8 changes for about 1/3 of the test suite files.

• tests/test_open_file.py
• tests/test_null_policy.py
• tests/test_json.py
• tests/test_header_items.py
• tests/test_example.py
• tests/test_enhancements.py

This pull request makes changes suggested by flake8. These are mainly the same types of changes in the various files:

• Remove unused sys import (replaced by tests/init.py)
• Reorganize library and 3rd-party imports
• Remove other unused imports
• Convert path lambdas to functions
• Change ambiguously short variable names to something more descriptive
• Add additional asserts.
  • Notes:
  • Most times these are useful by adding a bit more information about what is being tested.
  • A few times they don't add must to the test.

Test Results:

All test results continue to pass:

---------- coverage: python 3.10.9 -----------
Name                       Stmts   Miss  Cover
----------------------------------------------
lasio/__init__.py             28      6    79%
lasio/convert_version.py      20     20     0%
lasio/defaults.py             12      0   100%
lasio/examples.py             42     10    76%
lasio/excel.py                88     34    61%
lasio/exceptions.py            6      0   100%
lasio/las.py                 495     64    87%
lasio/las_items.py           220     30    86%
lasio/las_version.py          69     23    67%
lasio/reader.py              470     19    96%
lasio/writer.py              200      9    96%
----------------------------------------------
TOTAL                       1650    215    87%
Coverage XML written to file coverage.xml

--------------------------------------------------- benchmark: 1 tests --------------------------------------------------
Name (time in ms)                 Min       Max      Mean  StdDev    Median     IQR  Outliers     OPS  Rounds  Iterations
-------------------------------------------------------------------------------------------------------------------------
test_read_v12_sample_big     124.7772  131.6564  126.2386  2.2637  125.4748  1.1334       1;1  7.9215       8           1
-------------------------------------------------------------------------------------------------------------------------

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Let me know if this change could be accepted (or rejected) or needs some additional changes to be approved and merged.

Thank you, DC