Address mypy type errors

These are all situations where the type is potentially None but in
practice the value will not be None.

Where possible I've corrected the inferred type, and otherwise I've
protected with asserts if there is a guard elsewhere against None
values.

src/tld/base.py

@@ -1,5 +1,6 @@
 from codecs import open as codecs_open
 from urllib.request import urlopen
+from typing import Optional
 
 from .exceptions import (
     TldIOError,
@@ -17,9 +18,9 @@
 class BaseTLDSourceParser(metaclass=Registry):
     """Base TLD source parser."""
 
-    uid: str = None
-    source_url: str = None
-    local_path: str = None
+    uid: Optional[str] = None
+    source_url: str
+    local_path: str
 
     @classmethod
     def validate(cls):

src/tld/utils.py

@@ -200,7 +200,6 @@ def get_tld_names(
         ):
             return _tld_names
 
-        local_file = None
         try:
             # Load the TLD names file
             if isabs(cls.local_path):
@@ -441,6 +440,9 @@ def get_fld(
     if domain_parts is None:
         return None
 
+    # This should be None when domain_parts is None
+    # but mypy isn't quite smart enough to figure that out yet
+    assert non_zero_i is not None
     if non_zero_i < 0:
         # hostname = tld
         return parsed_url.hostname
@@ -497,6 +499,10 @@ def get_tld(
     if domain_parts is None:
         return None
 
+    # This should be None when domain_parts is None
+    # but mypy isn't quite smart enough to figure that out yet
+    assert non_zero_i is not None
+
     if not as_object:
         if non_zero_i < 0:
             # hostname = tld
@@ -507,6 +513,9 @@ def get_tld(
         # hostname = tld
         subdomain = ""
         domain = ""
+        # This is checked in process_url but the type is ambiguous (Optional[str])
+        # so this assertion is just to satisfy mypy
+        assert parsed_url.hostname is not None, "No hostname in URL"
         _tld = parsed_url.hostname
     else:
         subdomain = ".".join(domain_parts[:non_zero_i-1])