Bump six maximum required version to 3.16.0

See conan-io/conan-package-tools#526

When running `run_create_in_docker` fails currently with:
```Traceback (most recent call last):
  File "/opt/pyenv/versions/3.7.5/lib/python3.7/site-packages/pkg_resources/__init__.py", line 583, in _build_master
    ws.require(__requires__)
  File "/opt/pyenv/versions/3.7.5/lib/python3.7/site-packages/pkg_resources/__init__.py", line 900, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/opt/pyenv/versions/3.7.5/lib/python3.7/site-packages/pkg_resources/__init__.py", line 791, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.VersionConflict: (conan-package-tools 0.34.4 (/home/conan/.local/lib/python3.7/site-packages), Requirement.parse('conan-package-tools==0.34.3'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/pyenv/versions/3.7.5/bin/run_create_in_docker", line 6, in <module>
    from pkg_resources import load_entry_point
  File "/opt/pyenv/versions/3.7.5/lib/python3.7/site-packages/pkg_resources/__init__.py", line 3250, in <module>
    @_call_aside
  File "/opt/pyenv/versions/3.7.5/lib/python3.7/site-packages/pkg_resources/__init__.py", line 3234, in _call_aside
    f(*args, **kwargs)
  File "/opt/pyenv/versions/3.7.5/lib/python3.7/site-packages/pkg_resources/__init__.py", line 3263, in _initialize_master_working_set
    working_set = WorkingSet._build_master()
  File "/opt/pyenv/versions/3.7.5/lib/python3.7/site-packages/pkg_resources/__init__.py", line 585, in _build_master
    return cls._build_from_requirements(__requires__)
  File "/opt/pyenv/versions/3.7.5/lib/python3.7/site-packages/pkg_resources/__init__.py", line 598, in _build_from_requirements
    dists = ws.resolve(reqs, Environment())
  File "/opt/pyenv/versions/3.7.5/lib/python3.7/site-packages/pkg_resources/__init__.py", line 786, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'six<1.15.0,>=1.10.0' distribution was not found and is required by conan-package-tools
```

conans/requirements.txt

@@ -5,7 +5,7 @@ colorama>=0.3.3, <0.5.0
 PyYAML>=3.11, <6.0
 patch-ng>=1.17.4, <1.18
 fasteners>=0.14.1
-six>=1.10.0,<=1.15.0
+six>=1.10.0,<=1.16.0
 node-semver==0.6.1
 distro>=1.0.2, <=1.5.0
 future>=0.16.0, <0.19.0