[strawberry.ext.mypy_plugin] Fix import error for users who don't use pydantic #9505

Job	Run time
💻 Generate test matrix	31s
🪟 Tests on Windows	4m 25s
📈 Benchmarks	3m 30s
✨ Lint	1m 14s
🔬 Tests on 3.12	51s
🔬 Tests on 3.11	2m 4s
🔬 Tests on 3.10	1m 58s
🔬 Tests on 3.9	2m 13s
🔬 Tests on 3.8	1m 45s
🔬 Django tests on 3.11 (django=4.2.0)	1m 16s
🔬 Django tests on 3.11 (django=4.1.0)	1m 41s
🔬 Django tests on 3.11 (django=4.0.0)	1m 49s
🔬 Django tests on 3.11 (django=3.2.0)	1m 19s
🔬 Starlette tests on 3.11 (starlette=0.28.0)	1m 14s
🔬 Starlette tests on 3.11 (starlette=0.27.0)	1m 25s
🔬 Starlette tests on 3.11 (starlette=0.26.1)	1m 22s
🔬 Test integrations on 3.11 (integration=aiohttp)	1m 18s
🔬 Test integrations on 3.11 (integration=chalice)	1m 27s
🔬 Test integrations on 3.11 (integration=channels)	1m 41s
🔬 Test integrations on 3.11 (integration=fastapi)	1m 44s
🔬 Test integrations on 3.11 (integration=flask)	1m 36s
🔬 Test integrations on 3.11 (integration=sanic)	1m 28s
🔬 Test integrations on 3.11 (integration=starlite)	1m 37s
🔬 Pydantic tests on 3.11 (pydantic=1.10)	1m 37s
🔬 Pydantic tests on 3.11 (pydantic=2.0.3)	1m 24s
🔬 Pyright tests on 3.12	59s
🔬 Pyright tests on 3.11	2m 16s
🔬 Pyright tests on 3.10	1m 42s
🔬 Pyright tests on 3.9	1m 47s
🔬 Pyright tests on 3.8	1m 46s
🔬 CLI tests on 3.12	1m 8s
🔬 CLI tests on 3.11	1m 13s
🔬 CLI tests on 3.10	1m 11s
🔬 CLI tests on 3.9	1m 13s
🔬 CLI tests on 3.8	1m 30s
	57m 14s

Provide feedback