Synchronize requirements.txt and setup.py

Recently requirements.txt and setup.py have started to desync quite a bit. This PR attempts to bring them back in line.

### Notable changes:
* *Flask-Scrypt* and *Scrypt* dependencies removed (not used anymore after switch back to Werkzeug's default hashing)
* *wsgiref* dependency removed (not used anywhere)
* GitHub tarball link to Miguel Grinberg's fork of *Flask-WhooshAlchemy* removed, as those changes have since been merged into Flask-WhooshAlchemy itself (since v0.56)
* *pytest* version bumped to 2.6.0