Travis CI: Lint Python code for undefined names

[cclauss]

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.

[simonarvin]

Hi! The vimba Importer is designed to import camera frames from a Vimba-compatible camera. For offline tracking, use cv Importer instead.

Best, Simon

[cclauss]

Ahh... Now I see. Thanks. What about image and Frame?

flake8 testing of https://github.com/

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

./importers/vimba.py:24:33: F821 undefined name 'image'
        self.arm(width, height, image)
                                ^
./importers/vimba.py:27:36: F821 undefined name 'Frame'
    def acquire_frame(self, frame: Frame, delay: int = 1) -> None:
                                   ^
2     F821 undefined name 'image'
2

[simonarvin]

Right, thank you for noticing. Should be fixed now!

[cclauss]

Tests now pass.