Avoid SyntaxWarnings on Python >= 3.8
% `python3.8`
```
>>> "second" is "second"
<stdin>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
```
`flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics`
```
./Tools/autotest/arducopter.py:3899:20: F632 use ==/!= to compare constant literals (str, bytes, int, float, tuple)
if loop is not "second":
^
./Tools/autotest/arducopter.py:4047:20: F632 use ==/!= to compare constant literals (str, bytes, int, float, tuple)
if loop is not "second":
^
2 F632 use ==/!= to compare constant literals (str, bytes, int, float, tuple)
2
```
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