Eliminated _WriterBase class, removed unused fixtures from methods in pandas/io/excel/test_writers.py
- xref #26784
- passes `black pandas`
- passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Hello @dayoreke! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-10-13 21:31:37 UTC > Thanks for taking a look - this one might be tricky.

this addresses Eliminate _WriterBase class and engine and ext injected into tests https://github.com/pandas-dev/pandas/pull/26662#discussion_r291260131 from #26784

could split into two PRs if it makes it easier to review. test_basics_with_nan failing. Thanks @dayoreke Very nice!