The bsddb3 python package requires Berkeley DB installed. If using OSX, this can be done with brew install berkeley-db, but in order to install bsddb3, you'll need to execute the following (update for your current installed version):
export BERKELEYDB_INCDIR=/usr/local/Cellar/berkeley-db/5.3.21/include
export BERKELEYDB_LIBDIR=/usr/local/Cellar/berkeley-db/5.3.21/lib
export BERKELEYDB_DIR=/usr/local/Cellar/berkeley-db/5.3.21
pip install -r requirements.txt