Deprecate and undocument install_ext...
Deprecate and undocument install_ext
It seem to me that Python packaging has come up to a point where
actually telling peoples they can install python extensions not using
package manager is unreasonable.
Recent extension even recommend using `install_ext` as **the way** to
install things, which is unsecure, confusing, and there is no update
mechanisme, nor version conflict resolution.
