Require uproot to be version 3.0.0 or newer (#279)

3.0.0 introduced a change in the public API for accessing fSumw2. This
is needed to make sure that the code in pyhf/readxml.py works with
uproot.

setup.py

@@ -48,7 +48,7 @@
        'coverage>=4.0',  # coveralls
        'matplotlib',
        'jupyter',
-       'uproot',
+       'uproot>=3.0.0',
        'papermill',
        'graphviz',
        'sphinx',