fix: Use uproot3-methods with uproot3 (#1200)

* Use uproot3-methods namespace
   - Required by uproot3 since the release of v3.14.1 and the yank of v3.14.0

Author: matthewfeickert

src/pyhf/writexml.py

@@ -6,7 +6,7 @@
 import xml.etree.cElementTree as ET
 import numpy as np
 import uproot3 as uproot
-from uproot_methods.classes import TH1
+from uproot3_methods.classes import TH1
 
 from .mixins import _ChannelSummaryMixin