remove argparse from requirements.txt

[blshkv]

https://github.com/aboul3la/Sublist3r/blob/master/requirements.txt#L1

I think this line pulls the package called "argparse' unnecessarily. According to the official page,
https://pypi.org/project/argparse/ it has been unmaintained for quite some time now and was developed for old python compatibility only.

The PyPi argparse package is mostly for people who want to have argparse on
older Pythons, like < 2.7 or < 3.2 because it was not in stdlib back then.

python 2.7 and >3.4 supports "import argparse" natively using build-in functionality:
https://docs.python.org/3/howto/argparse.html

So you can simply drop that requirement.