MAINT: reinstate symeig but with deprecation decorator

Fully removing `symeig` in https://github.com/scipy/scipy/pull/3599 was premature, causing downstream problems e.g.
https://github.com/rmcgibbo/mixtape/issues/180
https://github.com/scikit-learn/scikit-learn/issues/3177

I've put it back in with a deprecation decorator.  However, I'm not able to see any deprecation warning when I use it, but maybe this is a separate issue.