Use pdfminer (pdfminer3k is no longer available on pip)

[TomTJarosz]

@felipeochoa it appears that pdfminer3k has been removed from pip https://pypi.org/project/pdfminer3k/ . Consequently, pip install minecart no longer works:

> pip install minecart
Collecting minecart
  Using cached minecart-0.3.0-py3-none-any.whl (23 kB)
Requirement already satisfied: six in /usr/local/share/c3/conda/envs/mc4/lib/python3.8/site-packages (from minecart) (1.14.0)
ERROR: Could not find a version that satisfies the requirement pdfminer3k (from minecart) (from versions: none)
ERROR: No matching distribution found for pdfminer3k (from minecart)

Note that pdfminer >=20191010 supports python 3 only https://pypi.org/project/pdfminer/

[coveralls]

Coverage decreased (-11.3%) to 51.555% when pulling 0c987784e5593f585d27df96620d5ac07db6444d on TomTJarosz:master into f39a3934e4be271f2cc061e44bee34c6d6cb4126 on felipeochoa:master.