Import performance part 3

In #406 we decided to get rid of scanpy.api, which worsened our import time. Thanks to @ivirshup (#703, #704), the main culprits to long import times are out of the game, but there’s still room for improvement.

I used profimp to identify the rest. I started with just profimp --html 'import scanpy', identified the external imports that take a while, and created a file in which I imported them before finally importing scanpy:

uvx --with=.[leiden] profimp --html "# anndata big imports
import numpy
import pandas
import zarr
import h5py

# scanpy big imports
import numba
import sklearn          # preprocessing._simple
#import sklearn.metrics  # neighbors
#import networkx         # diffmap, paga, plotting._utils
import leidenalg
import matplotlib.pyplot

# rest
import scanpy" | save -f dist/profimp-scanpy.html

Start by running the documented profimp command with the listed imports and inspect dist/profimp-scanpy.html. The issue provides no target files, tests, or specific optimization scope; completion would require identifying and reducing the remaining scanpy import-time costs.