Add python version requirement to frontpage and readme #3360

Job	Run time
lint	49s
format	45s
codecov (./packages/autogen-core)	54s
docs (./packages/autogen-core)	1m 22s
docs-example-check (./packages/autogen-core)	2m 37s
codecov (./packages/autogen-magentic-one)	1m 46s
codecov (./packages/autogen-ext)	2m 36s
codecov (./packages/autogen-agentchat)	1m 12s
mypy (./packages/autogen-core)	2m 3s
pyright (./packages/autogen-core)	1m 4s
mypy (./packages/autogen-magentic-one)	1m 19s
test (./packages/autogen-core)	1m 10s
mypy (./packages/agbench)	54s
pyright (./packages/autogen-magentic-one)	52s
test (./packages/autogen-magentic-one)	1m 30s
mypy (./packages/autogen-ext)	1m 57s
pyright (./packages/agbench)	1m 0s
test (./packages/autogen-ext)	2m 55s
mypy (./packages/autogen-agentchat)	1m 8s
pyright (./packages/autogen-ext)	59s
test (./packages/autogen-agentchat)	54s
mypy (./packages/magentic-one-cli)	1m 8s
pyright (./packages/autogen-agentchat)	1m 2s
pyright (./packages/magentic-one-cli)	50s
check-proto-changes-python	1m 1s
markdown-code-lint	2m 5s
samples-code-check	51s
	36m 43s

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