Convert PC Python objects into strings

[connorjward]

Makes it much more pleasant to construct preconditioners.

It really is that easy.

(Tests and Firedrake PR to follow)

[JHopeCollins]

I am very happy with this functionality!

One small request. Can we have a test that non-PC objects also get converted?
Doesn't have to set up a solver or anything, just something like:

class MyPythonSNES:
	pass

def test_python_type_option():
	with petsctools.inserted_options(
		parameters={
			"snes_type": "python",
			"snes_python_type": MyPythonSNES,
		}
	):
		assert PETSc.Options()["snes_python_type"] == f"{__name__}.MyPythonSNES", "Python type name was not substituted into the options dictionary"