MAINT: regex char class improve
* remove superfluous regex character classes
from the codebase (those that contain a single
character incur the overhead of a class with
none of the advantages of a class)

* for more details, see similar change in NumPy:
https://github.com/numpy/numpy/pull/18083

* check performed with some simple [scraping code](https://github.com/tylerjereddy/regex-improve)