Refactor pyexec (#1318)

This is some refactoring I did on the way towards resolving pyscript#1313.
I added a new _run_pyscript Python function which executes the code
inside a context manager that sets the display target. We can then
return a JS object wrapper directly from Python.

I moved the "installation" of the pyscript module to loadInterpreter,
and pyimport pyscript_py there and give it a type. This avoids a bunch
of creating and deleting of proxies for pyscript_py and allows us to
give it a type once and for all.

I also did some minor logic cleanup in a few places.

pyscriptjs/jest.config.js

@@ -1,6 +1,7 @@
 //jest.config.js
 module.exports = {
     preset: 'ts-jest',
+    setupFilesAfterEnv: ['./tests/unit/setup.ts'],
     testEnvironment: './jest-environment-jsdom.js',
     extensionsToTreatAsEsm: ['.ts'],
     transform: {