subprocess2: Ensure environment keys and values are strings on Python 3

Example error during "git cl upload":
Traceback (most recent call last):
  File "C:\Google\depot_tools\presubmit_support.py", line 220, in CallCommand
    returncode, stdout = self._RunWithTimeout(cmd, test.stdin, test.kwargs)
  File "C:\Google\depot_tools\presubmit_support.py", line 204, in _RunWithTimeout
    p = subprocess.Popen(cmd, **kwargs)
  File "C:\Google\depot_tools\subprocess2.py", line 143, in __init__
    super(Popen, self).__init__(args, **kwargs)
  File "C:\Google\depot_tools\bootstrap-3_8_0b1_chromium_1_bin\python\bin\Lib\subprocess.py", line 390, in __init__
    errread, errwrite)
  File "C:\Google\depot_tools\bootstrap-3_8_0b1_chromium_1_bin\python\bin\Lib\subprocess.py", line 640, in _execute_child
    startupinfo)
TypeError: environment can only contain strings

Bug: 984182
Change-Id: Id996d73a80845aaeaa061107ed123627091cb600
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1815872
Commit-Queue: Raul Tambre <raul@tambre.ee>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Raul Tambre <raul@tambre.ee>

subprocess2.py

@@ -121,6 +121,17 @@ class Popen(subprocess.Popen):
     env = get_english_env(kwargs.get('env'))
     if env:
       kwargs['env'] = env
+    if kwargs.get('env') is not None and sys.version_info.major != 2:
+      # Subprocess expects environment variables to be strings in Python 3.
+      def ensure_str(value):
+        if isinstance(value, bytes):
+          return value.decode()
+        return value
+
+      kwargs['env'] = {
+          ensure_str(k): ensure_str(v)
+          for k, v in kwargs['env'].items()
+      }
     if kwargs.get('shell') is None:
       # *Sigh*:  Windows needs shell=True, or else it won't search %PATH% for
       # the executable, but shell=True makes subprocess on Linux fail when it's

tests/subprocess2_test.py

@@ -81,6 +81,14 @@ class DefaultsTest(unittest.TestCase):
     mockCommunicate.assert_called_with(
         ['foo'], a=True, stdin=subprocess2.VOID_INPUT, stdout=subprocess2.PIPE)
 
+  @mock.patch('subprocess.Popen.__init__')
+  def test_env_type(self, mockPopen):
+    if sys.version_info.major != 2:
+      subprocess2.Popen(['foo'], env={b'key': b'value'})
+      mockPopen.assert_called_with(['foo'],
+                                   env={'key': 'value'},
+                                   shell=mock.ANY)
+
 
 def _run_test(with_subprocess=True):
   """Runs a tests in 12 combinations: