Remove Python 2 support

[shadchin]

Now import configargparse on Python 2 fail

Python 2.7.18 (default, Jul  1 2022, 12:27:04)
[GCC 9.4.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import configargparse
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "configargparse.py", line 406
    raise ValueError(f"Error trying to unquote the quoted string: {text}: {e}") from e
                                                                             ^
SyntaxError: invalid syntax
>>>

[bw2]

Thanks for catching this. Any downside to maintaining python2 support for as long as we can?

[bw2]

No test environment for 2.7 so going to remove it.