tests/fixtures: Fix FilesDef type to include bytes values

The build_files() helper which handles these FilesDef nested dicts
already has code to handle values of type 'bytes' (they get written
verbatim instead of first being processed by DALS()).

Changing the FilesDef type to reflect this prevents Mypy from failing
when encountering legitimate bytes values in these dicts.

tests/fixtures.py

@@ -83,8 +83,10 @@ def setUp(self):
 
 
 # Except for python/mypy#731, prefer to define
-# FilesDef = Dict[str, Union['FilesDef', str]]
-FilesDef = Dict[str, Union[Dict[str, Union[Dict[str, str], str]], str]]
+# FilesDef = Dict[str, Union['FilesDef', str, bytes]]
+FilesDef = Dict[str, Union[
+    Dict[str, Union[Dict[str, Union[str, bytes]], str, bytes]], str, bytes]
+]
 
 
 class DistInfoPkg(OnSysPath, SiteDir):