More Python/pip package cleanup : version 2.0.17.

Generate astronomy.py directly in the package directory.
I realized it doesn't make sense to generate it in the
parent directory and then copy it; just generate it where
it will end up anyway.

Updated documentation so people know they can just do

    pip install astronomy-engine

to install Astronomy Engine in their Python project.

Removed the GitHub Actions status badge because it is redundant with
the checkmark/X indicator.

Now that private symbols are no longer exported, I had to
fix a couple of places where the unit tests still accessed them.

source/python/astronomy/.gitignore

@@ -1 +0,0 @@
-astronomy.py