[u2c.py] Fix unicode files argument handling for Python 2.7

[KevinXuxuxu]

As mentioned in https://stackoverflow.com/questions/22947181/dont-argparse-read-unicode-from-commandline it is a known issue with Python 2.7 and argparse because Python 2.7 treats sys.argv as a list of byte strings, not Unicode strings, while Python 3 does. So when passing in files which contains unicode (Chinese, Japanese...) results in the following error:

$ ~/u2c.py -a wark copyparty.domain.name/movies/ 飞黄腾达.mp4 
/share/homes/xu/u2c.py:1520: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
  if "--version" in sys.argv:
/share/homes/xu/u2c.py:1524: UnicodeWarning: Unicode unequal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
  sys.argv = [x for x in sys.argv if x != "--ws"]
usage: u2c.py [-h] [-v] [-a PASSWD] [-s] [-x REGEX] [--ok] [--touch] [--ow]
               [--owo] [--spd] [--version] [-u] [-ud] [-uf PATH] [--cls]
               [--rh TRIES] [--dl] [--dr] [--drd] [--wsalt S] [--chs] [--jw]
               [-j CONNS] [-J CORES] [--sz MiB] [--szm MiB] [-nh] [-ns]
               [--cxp SEC] [--cd SEC] [--safe] [-z] [-te PATH] [-td]
               url files [files ...]
u2c.py: error: argument files: invalid unicode value: '\xe9\xa3\x9e\xe9\xbb\x84\xe8\x85\xbe\xe8\xbe\xbe.mp4'

This PR fixes this by explicitly specifying encoding when handling files arg. It's only effective for Python 2, no behavior change is expected for Python 3.

Note that this issue might happen to other arguments using type=unicode (e.g. url) but I suppose fixing files arg should be the most useful case. I haven't tested for other args yet.

This PR complies with the DCO; https://developercertificate.org/

[9001]

good catch! thanks :>