Small refactor to improve code readability

These changes are small improvements to improve code readability:
rename a variable to a more descriptive name (from `exclude_is_None`
to `using_default_exclude`), use a better syntax to include the type
annotation for `gitignore` variable (from typing comment to
Python-style typing annotation)

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>

src/black/__init__.py

@@ -628,9 +628,9 @@ def get_sources(
     sources: Set[Path] = set()
     root = ctx.obj["root"]
 
-    exclude_is_None = exclude is None
+    using_default_exclude = exclude is None
     exclude = re_compile_maybe_verbose(DEFAULT_EXCLUDES) if exclude is None else exclude
-    gitignore = None  # type: Optional[PathSpec]
+    gitignore: Optional[PathSpec] = None
     root_gitignore = get_gitignore(root)
 
     for s in src:
@@ -666,7 +666,7 @@ def get_sources(
 
             sources.add(p)
         elif p.is_dir():
-            if exclude_is_None:
+            if using_default_exclude:
                 p_gitignore = get_gitignore(p)
                 # No need to use p's gitignore if it is identical to root's gitignore
                 # (i.e. root and p point to the same directory).