Resolve "Add Pipfile to support Pipenv"

Pipfile was added to support pipenv, and the existing requirements.txt was updated. The required Python version is not specified in the Pipfile (it will be checked at runtime). Closes #65 (closed)

requirements.txt

 Jinja2>=2.10.1
 PyYAML>=5.3.1
-netaddr>=0.7
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+netaddr>=0.8
+-e git+https://gitlab.ics.muni.cz/muni-kypo-crp/backend-python/kypo-topology-definition.git#egg=kypo-topology-definition
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