Stop testing PyMongo with Python 2.6 in drivers-nightly

[2019/02/28 15:00:41.750] Running auth tests over nossl with python /opt/python/2.6/bin/python

[2019/02/28 15:00:41.758] + /opt/python/2.6/bin/python -c 'import sys; print(sys.version)'

[2019/02/28 15:00:41.758] + COVERAGE_OR_PYTHON=/opt/python/2.6/bin/python

[2019/02/28 15:00:41.758] + COVERAGE_ARGS=

[2019/02/28 15:00:41.758] + '[' -n '' -a CPython = CPython ']'

[2019/02/28 15:00:41.758] + /opt/python/2.6/bin/python setup.py clean

[2019/02/28 15:00:41.772] Traceback (most recent call last):

[2019/02/28 15:00:41.772]   File "setup.py", line 9, in <module>

[2019/02/28 15:00:41.772]     raise RuntimeError("Python version >= 2.7 required.")

[2019/02/28 15:00:41.772] RuntimeError: Python version >= 2.7 required.