FutureWarning for the 1.11 to begin the process of switching leiden backend

[ilan-gold]

See: #2865

[ivirshup]

I think we sorta already did this:

sc.tl.leiden(adata)
/mnt/workspace/repos/scanpy/scanpy/tools/_leiden.py:144: FutureWarning: Use of leidenalg is discouraged and will be deprecated in the future.  Please use `flavor="igraph"` `n_iterations=2` to achieve similar results.  `directed` must also be `False` to work with `igraph`'s implementation.
  warnings.warn(msg, FutureWarning)

However, I do think the warning could be reworked

• Stack level should be set so the right line is shown
• Message should be more like:

FutureWarning: In the future, the default backend for leiden will be igraph instead of leidenalg. To achieve the future defaults please pass: flavor="igraph" and n_iterations=2. directed must also be False to work with igraph's implementation.

Note that results will be slightly different with the changed backend.

• We should only warn once, instead of every time leiden is called. Here's a util we defined in anndata for this (I think stolen from pandas?)

def warn_once(msg: str, category: type[Warning], stacklevel: int = 1):
    warnings.warn(msg, category, stacklevel=stacklevel)
    # Prevent from showing up every time an awkward array is used
    # You'd think `'once'` works, but it doesn't at the repl and in notebooks
    warnings.filterwarnings("ignore", category=category, message=re.escape(msg))