Update Python dependencies (requests, python-twitter) (#3870)

* Update requests to 2.18.1 (was 2.14.2)

requests package unvendored all of its packages.

Dependencies:
  - certifi==2017.4.17 (up-to-date)
  - chardet==3.0.4 (was 2.3.0)
  - idna==2.5 (new / moved from requests)
  - urllib3==1.21.1 (new / moved from requests)

* Update Twitter notifier (tweet.py)

- Replace `oauth2` with `oauthlib`
- Use updated twitter API

* Install requests-oauthlib 0.8.0

New dependencies for the updated python-twitter.
Dependencies:
  - oauthlib==2.0.2 (new)
  - requests==2.18.1 (unchanged / up-to-date)

* Install python-twitter at 3.3



Dependencies:
  - future==0.16.0 <-- Not really needed, so not installed
  - requests==2.18.1 (unchanged / up-to-date)
  - requests-oauthlib==0.8.0 (unchanged / up-to-date)

* Remove old libs

- `oauth2` (replaced by `oauthlib` + `requests_oauthlib`)
- `pythontwitter` (replaced by newer version in `twitter` folder)