runcommand: is with a literal

[Peter Sabaini]

From Python 3.8 the interpreter will complain about using "is" with a literal:

https://docs.python.org/3.8/whatsnew/3.8.html#changes-in-python-behavior

The runcommand module uses this construct in three places:

thespian/runcommand.py:446: SyntaxWarning: "is" with a literal. Did you mean "=="?
if outmark is 'normal' else
thespian/runcommand.py:448: SyntaxWarning: "is" with a literal. Did you mean "=="?
if outmark is 'normal':
thespian/runcommand.py:450: SyntaxWarning: "is" with a literal. Did you mean "=="?
elif outmark is 'error':

Not sure of immediate impact but probably should be fixed for cleanliness

[Kevin Quick]

Thanks for the report and the patch!