Auto merge of #27004 - saschanaz:py3-env, r=jdm

Fix Py3 environment setting failures

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`python3 mach build -d` now proceeds to actual build. Since Gecko landed full Python 3 support, updating mozjs should allow us to drop Python 2 to build Servo. (I still see failures on other commands e.g. `test-tidy`.)

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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python/servo/command_base.py

@@ -131,17 +131,17 @@ def archive_deterministically(dir_to_archive, dest_archive, prepend_path=None):
 
 
 def normalize_env(env):
-    # There is a bug in subprocess where it doesn't like unicode types in
+    # There is a bug in Py2 subprocess where it doesn't like unicode types in
     # environment variables. Here, ensure all unicode are converted to
-    # binary. utf-8 is our globally assumed default. If the caller doesn't
-    # want UTF-8, they shouldn't pass in a unicode instance.
+    # native string type. utf-8 is our globally assumed default. If the caller
+    # doesn't want UTF-8, they shouldn't pass in a unicode instance.
     normalized_env = {}
     for k, v in env.items():
         if isinstance(k, six.text_type):
-            k = k.encode('utf-8', 'strict')
+            k = six.ensure_str(k, 'utf-8', 'strict')
 
         if isinstance(v, six.text_type):
-            v = v.encode('utf-8', 'strict')
+            v = six.ensure_str(v, 'utf-8', 'strict')
 
         normalized_env[k] = v