[examples] minimal version requirement run-time check in PL
This PR adds a run-time version check for PL. Via the warning for now. This is a follow up to https://github.com/huggingface/transformers/pull/7852#issuecomment-718144095

In the nature of development we don't constantly re-run `pip install -r requirements.txt` so often when a breaking change is introduced we have to signal to each other - hey, upgrade your PL or so. It'd be much simpler to let the program do this automatically for us.

for now one needs to update requirements.txt and the relevant .py files, but we could automate this to have one source to maintain - parse `requirements.txt` and pull the important min-version from there...

for now this is just a hardcoded plain check.

**My only suggestion is to make it an error** - there are too too many warnings in the test suite for someone to notice this yet another one - so I vote for making it an error.

@sshleifer, @sgugger, @LysandreJik 