[Python] GitHub templates: assign python team instead of individuals (grpc#39560)

I've configured auto-assignment for @grpc/grpc-python-team team, which
should give use more flexibility in routing the tickets.

Author: sergiitk

.github/ISSUE_TEMPLATE/bug_report_python.md

@@ -3,8 +3,7 @@ name: Report a gRPC Python bug
 about: Create a report to help us improve
 labels: kind/bug, priority/P2, lang/Python
 assignees:
-  - sergiitk
-  - sreenithi
+  - grpc/grpc-python-team
 
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.github/ISSUE_TEMPLATE/feature_request_python.md

@@ -3,8 +3,7 @@ name: Request a gRPC Python feature
 about: Suggest an idea for this project
 labels: kind/enhancement, priority/P2, lang/Python
 assignees:
-  - sergiitk
-  - sreenithi
+  - grpc/grpc-python-team
 
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