Fix deposit client error:

I've created a metadata file and I've tried to use this command to
upload my submission:

swh deposit upload --username ipol --password "mypassword" \
                   --archive mlheIPOL.tgz \
                   --collection 'ipol' \
                   --metadata metadata_test1.xml

However I get the following error:

  Traceback (most recent call last):
    File "/home/user/ipol/testSHsubmission/python3env/bin/swh",
  line 5, in <module>
      from swh.core.cli import main
    File
  "/home/user/ipol/testSHsubmission/python3env/lib/python3.5/site-
  packages/swh/core/cli/__init__.py", line 14, in <module>
      from ..sentry import init_sentry
    File
  "/home/user/ipol/testSHsubmission/python3env/lib/python3.5/site-
  packages/swh/core/sentry.py", line 14
      return f'{main_package}@{version}'
                                       ^
  SyntaxError: invalid syntax

This could happen to any other public swh subcommands.

tl; dr
f-string are python3.6 onwards. the user's venv is python3.5 thus the syntax error