TST: honor encoding in read_fwf for memory-mapped files
- [x] closes #23254
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry: not needed

`encoding` was already working for memory-mapped files in `read_fwf`.

Hello @twoertwein! 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:

* In the file [`pandas/tests/io/parser/test_read_fwf.py`](https://github.com/pandas-dev/pandas/blob/66666cb1ee59daff22a8dc79cddbab387ea21627/pandas/tests/io/parser/test_read_fwf.py):

> [Line 640:1](https://github.com/pandas-dev/pandas/blob/66666cb1ee59daff22a8dc79cddbab387ea21627/pandas/tests/io/parser/test_read_fwf.py#L640): [W293](https://duckduckgo.com/?q=pep8%20W293) blank line contains whitespace

 @twoertwein rebased this in the browser and i think introduced an issue, can you rebase and ping on green. thanks! thanks @twoertwein 