Problems with expiration under python 3.6/Scrapy 1.6

[dchaplinsky]

  File "/my/superproject/venv/lib/python3.6/site-packages/scrapy_httpcache/extensions/httpcache_storage.py", line 91, in _read_data
    if 0 < self.expiration_secs < (datetime.datetime.now - data['update_time']).seconds:
TypeError: unsupported operand type(s) for -: 'builtin_function_or_method' and 'datetime.datetime'

Seems like you've forgotten to put parenthesis. Can do PR if you want.