Bug 1620744 - Convert generate_mapfile.py to py3; r=firefox-build-system-reviewers,rstewart

Differential Revision: https://phabricator.services.mozilla.com/D65854

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozbuild/frontend/gyp_reader.py

@@ -94,7 +94,6 @@ def handle_actions(actions, context, action_overrides):
         g = context['GENERATED_FILES'][output]
         g.script = action_overrides[name]
         g.inputs = action['inputs']
-        g.py2 = True
 
 
 def handle_copies(copies, context):

security/generate_mapfile.py

@@ -25,7 +25,7 @@ def main(output, input):
     is_mingw = "WINNT" == buildconfig.substs['OS_ARCH'] and \
         buildconfig.substs.get('GCC_USE_GNU_LD')
 
-    with open(input, 'rb') as f:
+    with open(input, 'r', encoding='utf-8') as f:
         for line in f:
             line = line.rstrip()
             # On everything except MinGW, remove all lines containing ';-'