requirements.txt: Set future version to >= 0.16.0

This fixes swh-scheduler debian package build on buster.

See encountered error below:

I: pybuild base:217: cd /<<PKGBUILDDIR>>/.pybuild/cpython3_3.7_swh.scheduler/build; python3.7 -m pytest --postgresql-exec /usr/lib/postgresql/11/bin/pg_ctl -m 'not db and not fs'
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/pluggy/manager.py", line 267, in load_setuptools_entrypoints
    plugin = ep.load()
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2410, in load
    self.require(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2433, in require
    items = working_set.resolve(reqs, env, installer, extras=self.extras)
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 791, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.VersionConflict: (future 0.16.0 (/usr/lib/python3/dist-packages), Requirement.parse('future>=0.18.0'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/lib/python3/dist-packages/pytest.py", line 91, in <module>
    raise SystemExit(pytest.main())
  File "/usr/lib/python3/dist-packages/_pytest/config/__init__.py", line 58, in main
    config = _prepareconfig(args, plugins)
  File "/usr/lib/python3/dist-packages/_pytest/config/__init__.py", line 196, in _prepareconfig
    pluginmanager=pluginmanager, args=args
  File "/usr/lib/python3/dist-packages/pluggy/hooks.py", line 284, in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
  File "/usr/lib/python3/dist-packages/pluggy/manager.py", line 67, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "/usr/lib/python3/dist-packages/pluggy/manager.py", line 61, in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
  File "/usr/lib/python3/dist-packages/pluggy/callers.py", line 203, in _multicall
    gen.send(outcome)
  File "/usr/lib/python3/dist-packages/_pytest/helpconfig.py", line 93, in pytest_cmdline_parse
    config = outcome.get_result()
  File "/usr/lib/python3/dist-packages/pluggy/callers.py", line 80, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/usr/lib/python3/dist-packages/pluggy/callers.py", line 187, in _multicall
    res = hook_impl.function(*args)
  File "/usr/lib/python3/dist-packages/_pytest/config/__init__.py", line 675, in pytest_cmdline_parse
    self.parse(args)
  File "/usr/lib/python3/dist-packages/_pytest/config/__init__.py", line 845, in parse
    self._preparse(args, addopts=addopts)
  File "/usr/lib/python3/dist-packages/_pytest/config/__init__.py", line 799, in _preparse
    self.pluginmanager.load_setuptools_entrypoints("pytest11")
  File "/usr/lib/python3/dist-packages/pluggy/manager.py", line 273, in load_setuptools_entrypoints
    message="Plugin %r could not be loaded: %s!" % (ep.name, e),
pluggy.manager.PluginValidationError: Plugin 'pytest_swh_scheduler' could not be loaded: (future 0.16.0 (/usr/lib/python3/dist-packages), Requirement.parse('future>=0.18.0'))!
E: pybuild pybuild:341: test: plugin distutils failed with: exit code=1: cd /<<PKGBUILDDIR>>/.pybuild/cpython3_3.7_swh.scheduler/build; python3.7 -m pytest --postgresql-exec /usr/lib/postgresql/11/bin/pg_ctl -m 'not db and not fs'
dh_auto_test: pybuild --test --test-pytest -i python{version} -p 3.7 returned exit code 13
make: *** [debian/rules:10: build] Error 25
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2
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Migrated from D3472 (view on Phabricator)