Google App Engine Python 3.7 build error: `pip_download_wheels` returned code: 1

It looks like your requirements.txt file is UTF-16 encoded:

>>> b'p\x00y\x00t\x00z\x00=\x00=\x002\x000\x001\x008\x00.\x005\x00\r\x00'.decode('utf-16')
'pytz==2018.5\r'

How did you create this file?

You can use this short script should re-encode the file correctly:

with open('requirements.txt', 'rb') as source_file:
    contents = source_file.read()

with open('requirements.txt', 'w+b') as dest_file:
    dest_file.write(contents.decode('utf-16').encode('utf-8'))