DEPS: use importlib for backend discovery #41815

[fangchenli]

• closes DEPS: clarifying setuptools's status as a dependency  #41815

We still use pkg_resources in mpl tests. But it doesn't affect end users.

[jreback]

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

[jreback]

thanks @fangchenli