Fix ".constants.json" not found issue when using Pyinstaller to pack openpyxl.

hook-openpyxl.py

# When you're encountering following error.
# > IOError: [Errno 2] No such file or directory: 'C:\\Users\\username\\AppData\\Local\\Temp\\_MEI502322\\openpyxl\\.constants.json'
# 
# This solution tested under Python 2.7.10 and Pyinstaller 3.0.
# 
# Put this file to your script folder. 
# Add this hook to your distribution by
# > pyinstaller --onefile --additional-hooks-dir=. yourscript.py
#
from PyInstaller.utils.hooks import collect_data_files

# Instruct pyinstaller to collect data files from openpyxl package.
datas = collect_data_files('openpyxl')