MAINT Use multi-line code snippets in tests

Fix #71: Upgrade to Python 3.7

MAINT Fix and refactor tests/make_test_list.py

Avoid using UTF8ToString

MAINT: Rename python2js_int to _python2js

"int" meant "internal", but it's too easily confused with "integer"

The only places we were using it was for strings that are ASCII anyway,
so this is both more efficient and avoids this Chrome bug:

https://bugs.chromium.org/p/chromium/issues/detail?id=868404

Load multiple packages in pyodide.loadPackage

Package pytest

Also remove the (failing) tests with simultaneous package loading

`square` -> `cube` to avoid aliasing

This makes for a clearer & more convincing demo since `square` was already defined on the Python side.

Fix: Loading packages from custom URLs (part 1)