[python] Drop support for Python 3.8 (EOL 2024/10)

[johnkerl]

Split out from #1849, in particular #1849 (comment), as requested by @bkmartinjr.

Needs deprecation pre-work in various packages/organizations that depend on TileDB-SOMA. Not for immediate implementation; a longer-term tracking issue.

sc-41048

[johnkerl]

See also #2282 for two warnings we can get rid of once we drop 3.8 support

[johnkerl]

We also need to handle these:

FutureWarning: SparseDataset is deprecated and will be removed in late 2024. It has been replaced by the public classes CSRDataset and CSCDataset.

For instance checks, use `isinstance(X, (anndata.experimental.CSRDataset, anndata.experimental.CSCDataset))` instead.

cc @jp-dark

[johnkerl]

See also #1785

[johnkerl]

Re-closing; #1785 is open and suffices