Feature/upgrade singer python

Just tested this and I am getting

Traceback (most recent call last):
  File "/Users/robinjanssens/Envs/test/bin/tap-appsflyer", line 10, in <module>
    sys.exit(main())
  File "/Users/robinjanssens/Envs/test/lib/python3.7/site-packages/tap_appsflyer/__init__.py", line 461, in main
    do_sync()
  File "/Users/robinjanssens/Envs/test/lib/python3.7/site-packages/tap_appsflyer/__init__.py", line 443, in do_sync
    stream.sync() # pylint: disable=not-callable
  File "/Users/robinjanssens/Envs/test/lib/python3.7/site-packages/tap_appsflyer/__init__.py", line 282, in sync_installs
    if utils.strptime(record["attributed_touch_time"]) > bookmark:
  File "/Users/robinjanssens/Envs/test/lib/python3.7/site-packages/singer/utils.py", line 58, in strptime
    return datetime.datetime.strptime(dtime, DATETIME_PARSE)
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/_strptime.py", line 577, in _strptime_datetime
    tt, fraction, gmtoff_fraction = _strptime(data_string, format)
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/_strptime.py", line 359, in _strptime
    (data_string, format))
ValueError: time data '2019-05-10T02:29:14.000000Z' does not match format '%Y-%m-%dT%H:%M:%SZ'

It seems to be caused by this tap directly calling the deprecated strptime method in singer.

I'll add the fix in a separate PR.

singer-io / tap-appsflyer

Feature/upgrade singer python #15

Description of change

Manual QA steps

Risks

Rollback steps