Update openpyxl, get _get_column_letter from new location

OpenDataServices/cove#954

Author: Bjwebb

flattentool/input.py

@@ -15,7 +15,8 @@
 import traceback
 import datetime
 import pytz
-from openpyxl.utils import _get_column_letter, column_index_from_string
+from openpyxl.utils import column_index_from_string
+from openpyxl.utils.cell import _get_column_letter
 from flattentool.exceptions import DataErrorWarning
 from flattentool.lib import isint, parse_sheet_configuration
 

setup.py

@@ -1,7 +1,7 @@
 from setuptools import setup
 import sys
 
-install_requires = ['jsonref', 'schema', 'openpyxl>=2,<2.4', 'six', 'pytz', 'xmltodict']
+install_requires = ['jsonref', 'schema', 'openpyxl>=2.5', 'six', 'pytz', 'xmltodict']
 
 if sys.version < '3':
     install_requires.append('unicodecsv')