PIT+search_after became universally safe in Elasticsearch 7.12 by adding an automatic sort tiebreaker field when using PITs called `_shard_doc` but now we need to do feature detection to make sure we use the previous scroll method on Elasticsearch <7.12 clusters
* Adds build status sticker to README
* Adds Python version to test matrix
* Adds debug echo message
* Adds back Python 3.5.3 to test matrix
* Adds Python version to test matrix
* Adds back Python 3.5.3 to test matrix
* Adds Python 3.8 to test matrix