Fix license: poe build-develop works

Without this,

  File "/Users/dan/src/temporalio/sdk-python/.venv/lib/python3.13/site-packages/setuptools/config/pyprojecttoml.py", line 55, in validate
    raise ValueError(f"{error}\n{summary}") from None
ValueError: invalid pyproject.toml config: `project.license`.
configuration error: `project.license` must be valid exactly by one definition (2 matches found):

    - keys:
        'file': {type: string}
      required: ['file']
    - keys:
        'text': {type: string}
      required: ['text']

Author: dandavison

pyproject.toml

@@ -5,7 +5,7 @@ description = "Temporal.io Python SDK"
 authors = [{ name = "Temporal Technologies Inc", email = "sdk@temporal.io" }]
 requires-python = "~=3.9"
 readme = "README.md"
-license = "MIT"
+license = { file = "LICENSE" }
 keywords = [
     "temporal",
     "workflow",