DEPS: use importlib for backend discovery #41815
- [x] closes #41815

We still use `pkg_resources` in mpl tests. But it doesn't affect end users. 
> Performing static analysis using mypy
pandas/plotting/_core.py:1763: error: Item "None" of "Optional[Tuple[EntryPoint, ...]]" has no attribute "__iter__" (not iterable)  [union-attr]
pandas/plotting/_core.py:1766: error: Invalid self argument "EntryPoint" to attribute function "load" with type "Callable[[EntryPoint], Any]"  [misc]
Found 2 errors in 1 file (checked 1297 source files)
Performing static analysis using mypy DONE

 thanks @fangchenli 