Flake8 maintenance for test suite final set

Description:

This change covers flake8 changes for the 2nd 3rd of the test suite files. This is part of Flake8 maintenance for test suite https://github.com/kinverarity1/lasio/issues/561

When this pull-request merges, then #561 can be considered resolved.

tests/test_serialization.py
tests/test_speed.py
tests/test_stack_curves.py
tests/test_version.py
tests/test_wrapped.py
tests/test_write.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.

Flake8 Ignores:

Test data in test_write.py has lines with trailing white space. Test data in test_wrapped.py has lines with trailing white space and long lines. Flake8 should be configured to ignore those lines. Below is the suggested configuration for a .flake8 file.

[flake8]
per-file-ignores =
    test_wrapped.py:W291,E501
    test_write.py:W291

Test Results:

las.py started reporting an additional miss (from 63 to 64) but I haven't identified the before and after diff yet.

----------------------------------------------
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     124.3532  125.6853  125.1433  0.4513  125.2544  0.6507       2;0  7.9908       8           1
-------------------------------------------------------------------------------------------------------------------------

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

Thank you, DC

kinverarity1 / lasio