no longer install pydap for 'io' extras in py3.10 (#6965)

* no longer install pydap for 'io' extras in py3.10

* better issue

Author: mathause

setup.cfg

@@ -84,7 +84,7 @@ io =
     netCDF4
     h5netcdf
     scipy
-    pydap
+    pydap; python_version<"3.10" # see https://github.com/pydap/pydap/issues/268
     zarr
     fsspec
     cftime