Bug 1603634 - check if line is a decodable type r=aki,marco

Changes:

Due to changes in `subprocess.Popen` return values in python3, it makes sense to check first if the line is a decodable type prior to attempting such action.

This change will resolve the following failure when tests are run in python3:

```
======================================================================
2) ERROR: test_get_output_from_command (test_base_script.TestHelperFunctions)
----------------------------------------------------------------------
   Traceback (most recent call last):
    test/test_base_script.py line 370 in test_get_output_from_command
      contents = self.s.get_output_from_command(["bash", "-c", "cat %s" % self.temp_file])
    mozharness/base/script.py line 1658 in get_output_from_command
      line = line.decode("utf-8")
   AttributeError: 'str' object has no attribute 'decode'
   -------------------- >> begin captured logging << --------------------
   root: INFO: ConsoleLogger online at 20191215 00:24:05Z in /Users/egao/mozilla-central/testing/mozharness
   root: INFO: Run as .tox/py35-hg4.3/bin/nosetests -v --with-xunit --rednose --force-color
   root: INFO: Getting output from command: ['bash', '-c', 'cat test_dir/mozilla']
   root: INFO: Copy/paste: bash -c "cat test_dir/mozilla"
   root: INFO: Reading from file tmpfile_stdout
   root: INFO: Output received:
   --------------------- >> end captured logging << ---------------------
```

Differential Revision: https://phabricator.services.mozilla.com/D57225

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

testing/mozharness/mozharness/base/script.py

@@ -36,6 +36,8 @@ import zlib
 from contextlib import contextmanager
 from io import BytesIO
 
+from six import binary_type
+
 from mozprocess import ProcessHandler
 
 import mozinfo
@@ -1655,7 +1657,8 @@ class ScriptMixin(PlatformMixin):
                     for line in output_lines:
                         if not line or line.isspace():
                             continue
-                        line = line.decode("utf-8")
+                        if isinstance(line, binary_type):
+                            line = line.decode("utf-8")
                         self.log(' %s' % line, level=log_level)
                     output = '\n'.join(output_lines)
         if os.path.exists(tmp_stderr_filename) and os.path.getsize(tmp_stderr_filename):
@@ -1667,7 +1670,8 @@ class ScriptMixin(PlatformMixin):
             for line in errors.rstrip().splitlines():
                 if not line or line.isspace():
                     continue
-                line = line.decode("utf-8")
+                if isinstance(line, binary_type):
+                    line = line.decode("utf-8")
                 self.log(' %s' % line, level=return_level)
         elif p.returncode not in success_codes and not ignore_errors:
             return_level = ERROR