Add Pipfile + runtime.txt test

tests/venv/pipfile/py36/Pipfile

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+[[source]]
+url = "https://pypi.python.org/simple"
+verify_ssl = true
+name = "pypi"
+
+[packages]
+pypi-pkg-test = "*"
+
+[dev-packages]
+there = "*"
+
+[requires]
+python_version = "3.6"

tests/venv/pipfile/py36/README.rst

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+Python - Pipfile with python_version and runtime.txt
+----------------------------------------------------
+
+We are ignoring the runtime.txt if there is a Pipfile or Pipfile.lock available.
+And since `python_version = "3.6"` in the Pipfile, the `python-3.7` in
+runtime.txt should be ignored. Is it?

tests/venv/pipfile/py36/runtime.txt

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+python-3.7

tests/venv/pipfile/py36/verify

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+#!/usr/bin/env python
+import sys
+
+import pypi_pkg_test
+import there
+
+print(sys.version_info)
+assert sys.version_info[:2] == (3, 6)