use python3 evocations in tests (#270)

On some systems the `python` binary doesn't have to be present. This
enables running the testsuite on such systems.

Co-authored-by: Eugene Yurtsev <eugene@langchain.dev>

Author: MeggyCal

tests/test_client.py

@@ -24,12 +24,12 @@ async def test_multi_server_mcp_client(
     client = MultiServerMCPClient(
         {
             "math": {
-                "command": "python",
+                "command": "python3",
                 "args": [math_server_path],
                 "transport": "stdio",
             },
             "weather": {
-                "command": "python",
+                "command": "python3",
                 "args": [weather_server_path],
                 "transport": "stdio",
             },
@@ -105,7 +105,7 @@ async def test_multi_server_connect_methods(
     client = MultiServerMCPClient(
         {
             "math": {
-                "command": "python",
+                "command": "python3",
                 "args": [math_server_path],
                 "transport": "stdio",
             },
@@ -147,7 +147,7 @@ async def test_get_prompt():
     client = MultiServerMCPClient(
         {
             "math": {
-                "command": "python",
+                "command": "python3",
                 "args": [math_server_path],
                 "transport": "stdio",
             }