Potential missing dependency

[berislavlopac]

When I run my tests on the latest release (0.13.1) I'm getting the following error:

ImportError while importing test module '/home/berislav/Development/personal/pyotr/src/tests/test_server.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
src/tests/test_server.py:7: in <module>
    from pyotr.server import Application
src/pyotr/server.py:10: in <module>
    from openapi_core import create_spec
../../../.virtualenvs/pyotr/lib/python3.7/site-packages/openapi_core/__init__.py:3: in <module>
    from openapi_core.shortcuts import (
../../../.virtualenvs/pyotr/lib/python3.7/site-packages/openapi_core/shortcuts.py:4: in <module>
    from openapi_core.validation.request.shortcuts import (
../../../.virtualenvs/pyotr/lib/python3.7/site-packages/openapi_core/validation/request/shortcuts.py:10: in <module>
    from openapi_core.validation.request.validators import RequestValidator
../../../.virtualenvs/pyotr/lib/python3.7/site-packages/openapi_core/validation/request/validators.py:21: in <module>
    from openapi_core.validation.request.datatypes import (
../../../.virtualenvs/pyotr/lib/python3.7/site-packages/openapi_core/validation/request/datatypes.py:3: in <module>
    from werkzeug.datastructures import ImmutableMultiDict
E   ModuleNotFoundError: No module named 'werkzeug'

werkzeug is listed in setup.cfg under options.extras_require, but this error implies that it's used more generally?