Drop external argparse dependency

[rlue]

argparse has been part of the python stdlib since 2.7 & 3.2, which reached end-of-life on 2020/01/01 and 2016/02/20, respectively. It can safely be used without being specified as an external dependency.

Why can't we just keep it?

Including this dependency causes pip to always reinstall the (superfluous) library whenever upgrading ffsubsync:

$ pip install --upgrade ffsubsync
...
Collecting argparse
  Using cached argparse-1.4.0-py2.py3-none-any.whl (23 kB)
...

In practice, even though this package is installed, import argparse always uses the stdlib version instead:

$ python -c "import argparse; print(argparse)"
<module 'argparse' from '~/.pyenv/versions/3.10.2/lib/python3.10/argparse.py'>

This is not strictly a problem, but rather an annoyance.