Add type annotations to download.py and fetcher.py

Add the stub for the requests package (types-requests)
to requirements-tests.txt.

Add urllib3 to the ignored imports. The project seems
to have added type annotations already but has not
released a version including them yet.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

requirements-test.txt

@@ -13,3 +13,4 @@ isort
 pylint
 mypy
 bandit
+types-requests

setup.cfg

@@ -22,3 +22,6 @@ files =
 
 [mypy-securesystemslib.*]
 ignore_missing_imports = True
+
+[mypy-urllib3.*]
+ignore_missing_imports = True

tuf/ngclient/_internal/requests_fetcher.py

@@ -7,7 +7,7 @@
 
 import logging
 import time
-from typing import Iterator, Optional
+from typing import Dict, Iterator, Optional
 from urllib import parse
 
 # Imports
@@ -31,7 +31,7 @@ class RequestsFetcher(FetcherInterface):
  session per scheme+hostname combination.
  """
 
- def __init__(self):
+ def __init__(self) -> None:
  # http://docs.python-requests.org/en/master/user/advanced/#session-objects:
  #
  # "The Session object allows you to persist certain parameters across
@@ -46,7 +46,7 @@ def __init__(self):
  # improve efficiency, but avoiding sharing state between different
  # hosts-scheme combinations to minimize subtle security issues.
  # Some cookies may not be HTTP-safe.
- self._sessions = {}
+ self._sessions: Dict[str, requests.Session] = {}
 
  # Default settings
  self.socket_timeout: int = 4 # seconds
@@ -146,7 +146,7 @@ def _chunks(
  finally:
  response.close()
 
- def _get_session(self, url):
+ def _get_session(self, url: str) -> requests.Session:
  """Returns a different customized requests.Session per schema+hostname
  combination.
  """