More fixes for Python 3 compatibility

[cclauss]

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[julianoes]

Thanks a lot @cclauss. We should get this in right after the release!

[julianoes]

@cclauss I've rebased this, fixed the conflict and force-pushed. Let's see what CI says.

[julianoes]

Thanks @cclauss!

[cclauss]

There are still three undefined name...

flake8 testing of https://github.com/PX4/Firmware on Python 3.8.0

$ flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics

./Tools/uorb_graph/create.py:369:70: F821 undefined name 'path'
                print('Failed reading file: %s, skipping content.' % path)
                                                                     ^
./platforms/nuttx/Debug/Nuttx.py:419:14: F821 undefined name 'regmap'
		for reg in regmap:
             ^
./platforms/nuttx/Debug/Nuttx.py:420:24: F821 undefined name 'regmap'
			hex_addr= hex(int(a[regmap[reg]]))
                       ^
3     F821 undefined name 'path'
3

https://flake8.pycqa.org/en/latest/user/error-codes.html

On the flake8 test selection, this PR does not focus on "style violations" (the majority of flake8 error codes that psf/black can autocorrect). Instead these tests are focus on runtime safety and correctness:

• E9 tests are about Python syntax errors usually raised because flake8 can not build an Abstract Syntax Tree (AST). Often these issues are a sign of unused code or code that has not been ported to Python 3. These would be compile-time errors in a compiled language but in a dynamic language like Python they result in the script halting/crashing on the user.
• F63 tests are usually about the confusion between identity and equality in Python. Use ==/!= to compare str, bytes, and int literals is the classic case. These are areas where a == b is True but a is b is False (or vice versa). Python >= 3.8 will raise SyntaxWarnings on these instances.
• F7 tests logic errors and syntax errors in type hints
• F82 tests are almost always undefined names which are usually a sign of a typo, missing imports, or code that has not been ported to Python 3. These also would be compile-time errors in a compiled language but in Python a NameError is raised which will halt/crash the script on the user.

[julianoes]

Thanks @cclauss, see #13763.

[julianoes]

It's odd that this now fails in CI after all:

[2019-12-19T10:21:36.058Z] + python msg/tools/uorb_to_ros_rtps_ids.py -i msg/tools/uorb_rtps_message_ids.yaml -o px4_ros_com/templates/uorb_rtps_message_ids.yaml
[2019-12-19T10:21:36.327Z] Traceback (most recent call last):
[2019-12-19T10:21:36.327Z]   File "msg/tools/uorb_to_ros_rtps_ids.py", line 11, in <module>
[2019-12-19T10:21:36.327Z]     import six
[2019-12-19T10:21:36.327Z] ImportError: No module named six
script returned exit code 1