test if all extensions support parallel install before attempting parallel extension install

[branfosj]

fixes #4897, closes #4948

[boegel]

Instead of this workaround/hack, I think we can implement a more direct lightweight check to see if all extensions support installing in parallel.

It should basically boil down to checking whether the install_extension_async method that will be used is the default Extension.install_extension_async method (which will raise NotImplementedError) or whether it's a customized version of it (which in practice should mean that the extension supports parallel installation).

Then it's a simply if/else rather than a try/except, and there's no room for calling the prepare method twice...

[branfosj]

Instead of this workaround/hack, I think we can implement a more direct lightweight check to see if all extensions support installing in parallel.

It should basically boil down to checking whether the install_extension_async method that will be used is the default Extension.install_extension_async method (which will raise NotImplementedError) or whether it's a customized version of it (which in practice should mean that the extension supports parallel installation).

Then it's a simply if/else rather than a try/except, and there's no room for calling the prepare method twice...

Done

[boegel]

lgtm

testing with matplotlib (doesn't support installing in parallel) + R and R-bundle-* on top, just to make sure...