Parsearg

+ test down to Python 3.6
+ relax restriction in setup.py to >= 3.6

+ tweak to quickstart-todos.py in README.md
+ edits to ./doc/examples/a*.py
+ edits to ./doc/README.org
+ re-ran ./doc/README.pdf

+ edits to ./doc/README.org
+ re-ran ./doc/README.pdf

+ restructured:

- ./examples -> ./doc/examples
- ./tests/conftest.py contents -> ./src/parsearg/example_trees.py

+ edits to README.md, ./doc/README.org

+ restructured:

- ./parsearg/examples -> ./examples
- ./parsearg/tests -> ./tests
- ./parsearg -> ./src/parsearg

+ adjusted examples and tests to match restructuring
+ tweaked setup.py

+ README.md at top level (contains Quickstart)
+ moved README.org within ./doc