Don't add Python Plugins to Manager Twice

[JeffersGlass]

Python plugins were being registered with the App's plugin manager twice: once in fetchPythonPlugins:

pyscript/pyscriptjs/src/main.ts

Lines 294 to 297 in c0f36aa

	if (typeof module.plugin !== 'undefined') {
	const py_plugin = module.plugin;
	py_plugin.init(this);
	this.plugins.addPythonPlugin(py_plugin);

And once in the init method of the Python Plugin class itself:

pyscript/pyscriptjs/src/python/pyscript.py

Lines 498 to 500 in c0f36aa

	def init(self, app):
	self.app = app
	self.app.plugins.addPythonPlugin(create_proxy(self))

This PR removes the second registration, and improves test_execution_hooks to check that the specified hooks are called exactly once, instead of at least once.

Fixes #1049

[FabioRosado]

Nice! Thank you for fixing this 😄