loosen pytz requirement

The updates to pytz have just been timezone db changes, which are important changes
to bring in. It'd be great to be able to pin applications against specific versions,
rather than have it enforced by singer-python

Author: jarobe42

setup.py

@@ -10,7 +10,7 @@
       classifiers=['Programming Language :: Python :: 3 :: Only'],
       url="http://singer.io",
       install_requires=[
-          'pytz==2018.4',
+          'pytz>=2018.4',
           'jsonschema==2.6.0',
           'simplejson==3.11.1',
           'python-dateutil>=2.6.0',