pytest找不到pip模块

我有看起来像这样的项目结构

\project
    \app.py
    \tests
        \__init__.py
        \test_startup.py

app.py looks like this

from starlette.applications import Starlette
from starlette.responses import UJSONResponse
from starlette.routing import Route


async def homepage(request):
    return UJSONResponse({'hello': 'world'})


app = Starlette(debug=True, routes=[
    Route('/', homepage)
])

test_startup.py looks like this

from starlette.testclient import TestClient

from ..app import app


def test_app():
    client = TestClient(app)
    response = client.get('/')
    assert response.status_code == 200

__init__.py is an empty file.

When I try to run pytest -v from my project directory it fails with the error

tests/test_startup.py:1: in <module>
    from starlette.testclient import TestClient
E   ModuleNotFoundError: No module named 'starlette'

I am able to run the application. Also I was trying to put conftest.py into both - tests and project folders and it did not help.

问题是什么?

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