Python: add compatibility layer for Python 2 & <=3.11

[Mingun]

Use the non-deprecated method in Python 3 and fix tests in Python >=3.12:

• Python 2 has only assertRaisesRegexp
• Python <=3.11 have both assertRaisesRegex and (deprecated) assertRaisesRegexp
• Python >=3.12 has only assertRaisesRegex

Should fix the following tests on https://ci.kaitai.io/

• EofExceptionU4
• EosExceptionBytes
• EosExceptionU4
• ExprBytesNonLiteral

Closes kaitai-io/kaitai_struct#1104

[GreyCat]

Sounds good, appreciate your fix here!

Do you think we can actually extract this into a separate file not to repeat these lines everywhere?

Something akin to helpers/javascript?

[Mingun]

I was thinking about building a base class for tests, but since I don't test it (I'm too lazy to set up all the language environments that will only be needed for one thing), I was not sure that this will work and I made the simplest solution.

Something like this, but I'm not sure if constructor should have some arguments:

class BaseTest(unittest.TestCase):
  def __init__():
    if not hasattr(unittest.TestCase, 'assertRaisesRegex'):
      setattr(unittest.TestCase, 'assertRaisesRegex', unittest.TestCase.assertRaisesRegexp)

[Mingun]

Ok, I found a way to run python tests on Windows (7):

• Python 3.10 was already installed
• run pip install xmlrunner (because it was missed)
• change path delimiter from : to ; in ci-python
• run ./ci-python in Git Bash

I implemented the check in a common file which was placed to the spec/python/extra folder where other such files are located (should we rename spec/python/extra to helpers/python?).

Running tests reports two classes of warnings, I've fix them too.