compiler: Run isort on compiled code (#355)

src/betterproto/templates/template.py.j2

@@ -16,6 +16,9 @@ from typing import {% for i in output_file.typing_imports|sort %}{{ i }}{% if no
 
 import betterproto
 from betterproto.grpc.grpclib_server import ServiceBase
+{% for i in output_file.imports|sort %}
+{{ i }}
+{% endfor %}
 {% if output_file.services %}
 import grpclib
 {% endif %}
@@ -216,7 +219,3 @@ class {{ service.py_name }}Base(ServiceBase):
         }
 
 {% endfor %}
-
-{% for i in output_file.imports|sort %}
-{{ i }}
-{% endfor %}