python(chore): ping example

python/examples/ping/.env-example

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+SIFT_API_KEY=
+BASE_URI=

python/examples/ping/main.py

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+import os
+
+from dotenv import load_dotenv
+from sift.ping.v1.ping_pb2 import PingRequest
+from sift.ping.v1.ping_pb2_grpc import PingServiceStub
+from sift_py.grpc.transport import SiftChannelConfig, use_sift_channel
+
+if __name__ == "__main__":
+    load_dotenv()
+
+    apikey = os.getenv("SIFT_API_KEY")
+    assert apikey
+
+    uri = os.getenv("BASE_URI")
+    assert uri
+
+    channel_config: SiftChannelConfig = {
+        "apikey": apikey,
+        "uri": uri,
+    }
+
+    with use_sift_channel(channel_config) as channel:
+        response = PingServiceStub(channel).Ping(PingRequest())
+        print(response)

python/examples/ping/requirements.txt

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+python-dotenv
+sift-stack-py==0.3.0rc3