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
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