Attempt to improve hardcoded password checks for strings

[alanverresen]

This pull request attempts to improve the hardcoded password checks for strings:

• overhauls the hardcoded_password_string() function
  • adds checks for annotated assignments
  • refactors checks for regular assignments and dictionaries
  • simplifies checks for comparisons and covers more cases
• overhauls test examples used to test plugin
  • reorganizes file to improve its structure
  • adds more examples to thoroughly test checks
  • adds tests based on examples of past GitHub issues for regression testing

This pull request also closes several issues:

• resolves False Negative under Python 3, reported if scanned under python 2.7 #551, which was probably already resolved
• resolves False negative for hardcoded credentials (B105) using python 3.8 #605, which was probably already resolved
• resolves hardcoded_password_string is not triggerend when using python typing #642 by adding checks for annotated assignments