Moved from Pipenv to Poetry

All the Blender Institute Python webprojects are moving to Poetry; it works
better than Pipenv in various regards.

tests/test_mypy.py

@@ -1,4 +1,5 @@
 import pathlib
+import sys
 import unittest
 
 import mypy.api
@@ -8,6 +9,13 @@ import blender_asset_tracer
 
 class MypyRunnerTest(unittest.TestCase):
     def test_run_mypy(self):
+        # This test doesn't work with Tox, it raises an AssertionError:
+        # /path/to/blender-asset-tracer/.tox/py37/lib/python3.7/site-packages is in the PYTHONPATH.
+        # Please change directory so it is not.
+        for path in sys.path:
+            if '/.tox/' in path and path.endswith('/site-packages'):
+                self.skipTest("Mypy doesn't like Tox")
+
         path = pathlib.Path(blender_asset_tracer.__file__).parent
         result = mypy.api.run(['--incremental', '--ignore-missing-imports', str(path)])