Remove more Python 2 compatibility code

- os.environ is always `str` in Python 3.
- The only string type is `str` so we can stop using `six.str_types`.
- `iteritems()` isn't necessary because dicts have the `items()` method.

python/tidy/servo_tidy/tidy.py

@@ -20,12 +20,10 @@ import subprocess
 import sys
 
 import colorama
-import six
 import toml
 import voluptuous
 import yaml
 from .licenseck import OLD_MPL, MPL, APACHE, COPYRIGHT, licenses_toml, licenses_dep_toml
-from six import iteritems
 topdir = os.path.abspath(os.path.dirname(sys.argv[0]))
 wpt = os.path.join(topdir, "tests", "wpt")
 
@@ -121,7 +119,7 @@ def is_iter_empty(iterator):
 
 
 def normilize_paths(paths):
-    if isinstance(paths, six.string_types):
+    if isinstance(paths, str):
         return os.path.join(*paths.split('/'))
     else:
         return [os.path.join(*path.split('/')) for path in paths]
@@ -374,7 +372,7 @@ def check_lock(file_name, contents):
         if name not in packages_by_name:
             yield (1, "duplicates are allowed for `{}` but it is not a dependency".format(name))
 
-    for (name, packages) in iteritems(packages_by_name):
+    for (name, packages) in packages_by_name.items():
         has_duplicates = len(packages) > 1
         duplicates_allowed = name in exceptions
 
@@ -419,7 +417,7 @@ def check_lock(file_name, contents):
                     visited_whitelisted_packages[dependency_name][package_name] = True
 
     # Check if all the exceptions to blocked packages actually depend on the blocked package
-    for dependency_name, package_names in iteritems(blocked_packages):
+    for dependency_name, package_names in blocked_packages.items():
         for package_name in package_names:
             if not visited_whitelisted_packages[dependency_name].get(package_name):
                 fmt = "Package {} is not required to be an exception of blocked package {}."