Bug 1614146 - Stop using byte literals in _hg_cleanenv. r=firefox-build-system-reviewers,glandium

Differential Revision: https://phabricator.services.mozilla.com/D62398

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

python/mozboot/mozboot/base.py

@@ -531,9 +531,9 @@ class BaseBootstrapper(object):
         string forces that no user or system hgrc file is used.
         """
         env = os.environ.copy()
-        env[b'HGPLAIN'] = b'1'
+        env['HGPLAIN'] = '1'
         if not load_hgrc:
-            env[b'HGRCPATH'] = b''
+            env['HGRCPATH'] = ''
 
         return env