chore(pyproject.toml): change license to text (#265)

`pip show a2a-sdk` was including the whole license file.

Changed to just the license name

Author: zeroasterisk

pyproject.toml

@@ -3,10 +3,10 @@ name = "a2a-sdk"
 dynamic = ["version"]
 description = "A2A Python SDK"
 readme = "README.md"
-license = { file = "LICENSE" }
+license = "Apache-2.0"
 authors = [{ name = "Google LLC", email = "googleapis-packages@google.com" }]
 requires-python = ">=3.10"
-keywords = ["A2A", "A2A SDK", "A2A Protocol", "Agent2Agent"]
+keywords = ["A2A", "A2A SDK", "A2A Protocol", "Agent2Agent", "Agent 2 Agent"]
 dependencies = [
   "fastapi>=0.115.2",
   "httpx>=0.28.1",