Fix for #459 (pydantic code gen only) (#460)

src/betterproto/templates/template.py.j2

@@ -7,7 +7,11 @@ import {{ i }}
 {% endfor %}
 
 {% if output_file.pydantic_dataclasses %}
-from pydantic.dataclasses import dataclass
+from typing import TYPE_CHECKING
+if TYPE_CHECKING:
+    from dataclasses import dataclass
+else:
+    from pydantic.dataclasses import dataclass
 {%- else -%}
 from dataclasses import dataclass
 {% endif %}