Louvain package is deprecated in favor of leidenalg

[scottgigante]

• [X] I have checked that this issue has not already been reported.
• [X] I have confirmed this bug exists on the latest version of scanpy.
• [X] (optional) I have confirmed this bug exists on the master branch of scanpy.

---

import warnings
warnings.simplefilter('always')
import scanpy

/home/scottgigante/.local/lib/python3.6/site-packages/scanpy/tools/_louvain.py:17: DeprecationWarning: This package has been superseded by the `leidenalg` package and will no longer be maintained. Please upgrade to the `leidenalg` package.
  from louvain.VertexPartition import MutableVertexPartition

Versions

``` ----- anndata 0.7.3 scanpy 1.6.0 sinfo 0.3.1 ----- PIL 7.2.0 anndata 0.7.3 apport_python_hook NA cffi 1.14.1 colorama 0.3.7 cycler 0.10.0 cython_runtime NA dateutil 2.8.1 get_version 2.1 h5py 2.10.0 igraph 0.8.2 importlib_metadata 1.6.1 joblib 0.16.0 kiwisolver 1.2.0 legacy_api_wrap 1.2 llvmlite 0.33.0 louvain 0.7.0 matplotlib 3.3.0 mpl_toolkits NA natsort 7.0.1 numba 0.50.1 numexpr 2.7.1 numpy 1.19.1 packaging 20.4 pandas 1.0.5 pkg_resources NA psutil 5.7.2 pyparsing 2.4.7 pytz 2020.1 scanpy 1.6.0 scipy 1.5.2 setuptools_scm NA sinfo 0.3.1 sitecustomize NA six 1.15.0 sklearn 0.23.1 tables 3.6.1 texttable 1.6.2 wcwidth 0.2.5 yaml 3.12 zipp NA zope NA ----- Python 3.6.9 (default, Nov 7 2019, 10:44:02) [GCC 8.3.0] Linux-4.4.0-18362-Microsoft-x86_64-with-Ubuntu-18.04-bionic 8 logical CPU cores, x86_64 ----- Session information updated at 2020-08-18 15:10 ```

[ivirshup]

@flying-sheep, can we keep the type annotation, but defer the import of louvain until it's called? That should prevent the warning on import of scanpy.