setup: bump python-dateutil minimum version to 2.1.0 to get py3 support Avoid py2 syntax: E File "/home/mk/kallithea-py3/build/minimum-dependency-versions-venv/lib/python3.7/site-packages/dateutil/tz.py", line 78 E `self._name`, E ^ E SyntaxError: invalid syntax

pytest >= 4.6.6, < 4.7
pytest-sugar >= 0.9.2, < 0.10
pytest-benchmark >= 3.2.2, < 3.3
pytest-localserver >= 0.5.0, < 0.6
mock >= 3.0.0, < 3.1
Sphinx >= 1.8.0, < 1.9
WebTest >= 2.0.3, < 2.1
isort == 4.3.21