Type annotations in metadata mappings

Add type annotations to metadata mappings (#2259)

This patch adds type annotations to metadata mappings so that the mypy type checker can detect bugs in code. While working on the path, I also encountered these issues:

1. mypy raises error on detecting for implicit None returns, so I had to add return None at many places.

2. At a few places, @property s' are accessed using cls. mypy rightfully raises an error, so I had to add ignore to silence it for now. It appears Python 3.9+ supports class level properties so the ignores could be replaced by the proper handling when the minimum python version is raised.

Closes #2259.

---

Migrated from D7342 (view on Phabricator)

swh/indexer/metadata_dictionary/__init__.py

 import collections
+from typing import DefaultDict, Dict, Set, Type
 
 import click
+from typing_extensions import Final
 
 from . import cff, codemeta, maven, npm, python, ruby
+from .base import BaseMapping
 
-MAPPINGS = {
+MAPPINGS: Final[Dict[str, Type[BaseMapping]]] = {
     "CodemetaMapping": codemeta.CodemetaMapping,
     "MavenMapping": maven.MavenMapping,
     "NpmMapping": npm.NpmMapping,
@@ -14,7 +17,7 @@ MAPPINGS = {
 }
 
 
-def list_terms():
+def list_terms() -> DefaultDict[str, Set[Type[BaseMapping]]]:
     """Returns a dictionary with all supported CodeMeta terms as keys,
     and the mappings that support each of them as values."""
     d = collections.defaultdict(set)
@@ -27,7 +30,7 @@ def list_terms():
 @click.command()
 @click.argument("mapping_name")
 @click.argument("file_name")
-def main(mapping_name, file_name):
+def main(mapping_name: str, file_name: str) -> None:
     from pprint import pprint
 
     with open(file_name, "rb") as fd: