Improve Pipfile + requirements.txt test

Previously we did not check that requirements.txt was ignored properly,
now we do.

tests/venv/pipfile/requirements-txt/Pipfile

@@ -4,7 +4,7 @@ verify_ssl = true
 name = "pypi"
 
 [packages]
-requests = "*"
+pypi-pkg-test = "*"
 
 [dev-packages]
 there = "*"

tests/venv/pipfile/requirements-txt/verify

@@ -1,3 +1,11 @@
 #!/usr/bin/env python
-import requests
+import pypi_pkg_test
 import there
+
+try:
+    import numpy
+except ImportError:
+    # We want an ImportError to be thrown for this test to pass
+    pass
+else:
+    raise Exception("'numpy' shouldn't have been installed! It was listed in requirements.txt but not in the Pipfile that has precedence.")