feat: Improvements to tox and Makefile due to issue on MacOS

[javydekoning]

I'm running into this issue on my Mac:

https://apple.stackexchange.com/questions/249313/mac-terminal-find-command-what-does-a-double-slash-in-the-result-output-mean

The problem is the find command isn't excluding the .aws-sam/ directory.

find src/ -iname "*.py" -not -path "src/.aws-sam/*"
...
src//.aws-sam/build/OrganizationHandler/handler.py
src//.aws-sam/build/OrganizationHandler/cfn_custom_resource/decorator.py
src//.aws-sam/build/OrganizationHandler/cfn_custom_resource/__init__.py
src//.aws-sam/build/OrganizationHandler/main.py

This can easily be fixed by removing the trailing slash.

find src -iname "*.py" -not -path "src/.aws-sam/*"
src/lambda_codebase/organization/handler.py
src/lambda_codebase/organization/main.py
src/lambda_codebase/event.py
src/lambda_codebase/account_bootstrap.py
src/lambda_codebase/generic_account_config.py
src/lambda_codebase/account_processing/configure_account_ou.py
src/lambda_codebase/account_processing/tests/test_account_creation.py
src/lambda_codebase/account_processing/tests/__init__.py

I'd also like to include the cfn-lint and yamllint execution.