Replace `inspect` module for parsing

We currently use the inspect module to extract Django code and structure.

This has several drawbacks:

The version of Django that we're parsing needs to be installed in the project (at least locally).
The inspect module executes code (on import) when parsing modules. This means:
- We can only inspect code we trust.
- Python code inspected can mutate itself on import (see #181 (comment) and #184 )

We recently refactored the populate_cbv script to abstract the importer into a protocol that will (hopefully) make it easier for new importers to be written. LibCSV may be an appropriate alternative to inspect.

ccbv/cbv/importer/importers.py

Lines 29 to 31 in a89118b

    
           class CodeImporter(Protocol): 
        
               def generate_code_data(self) -> Iterator[CodeElement]: 
        
                   ...

Python's ast module has potential, if we can be sure it preserves formatting.

https://github.com/PyCQA/astroid is interesting.

	class CodeImporter(Protocol):
	def generate_code_data(self) -> Iterator[CodeElement]:
	...