remove duplication of version in __init__ and remove not used variables

[alan-barzilay]

probably something like:
__version__ = importlib.metadata.version('pipreqs')

refs:
https://docs.python.org/3/library/importlib.html
python-poetry/poetry#273

[alan-barzilay]

actually, this might be better:

import importlib.metadata
import tomllib

try:
    with open("pyproject.toml", "rb") as f:
        pyproject = tomllib.load(f)
    __version__ = pyproject["tool"]["poetry"]["version"]
except Exception as e:
    __version__ = importlib.metadata.version('pipreqs')

(tomlib is new from version 3.11)

[alan-barzilay]

this may be useful in #411