Handle 2to3 outisde of setup.py

[ImNtReal]

Since Python 2 has been deprecated for a while, I ran 2to3 against pushover.py and removed the deprecated use_2to3 flag in setup.py.

[wilsonwang371]

@Thibauth can we get the merged ??

[m-gaster]

Not sure if this is actively maintained but I'm still getting this error. using uv

➜ uv add "python-pushover @ git+https://github.com/Thibauth/python-pushover"
 Updated https://github.com/Thibauth/python-pushover (420bde9)
  × Failed to build `python-pushover @ git+https://github.com/Thibauth/python-pushover`
  ├─▶ The build backend returned an error
  ╰─▶ Call to `setuptools.build_meta:__legacy__.build_wheel` failed (exit status: 1)

      [stderr]
      error in python-pushover setup command: use_2to3 is invalid.

      hint: This usually indicates a problem with the package or the build environment.