Bump Python minimum version to 3.9+ (github#151)

* Initial plan

* Update Python minimum version requirement to 3.9+

Co-authored-by: friggeri <106686+friggeri@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: friggeri <106686+friggeri@users.noreply.github.com>

Author: Copilot

python/README.md

@@ -217,5 +217,5 @@ Note: `assistant.message` and `assistant.reasoning` (final events) are always se
 
 ## Requirements
 
-- Python 3.8+
+- Python 3.9+
 - GitHub Copilot CLI installed and accessible

python/pyproject.toml

@@ -7,7 +7,7 @@ name = "github-copilot-sdk"
 version = "0.1.0"
 description = "Python SDK for GitHub Copilot CLI"
 readme = "README.md"
-requires-python = ">=3.8"
+requires-python = ">=3.9"
 license = {text = "MIT"}
 authors = [
     {name = "GitHub", email = "opensource@github.com"}
@@ -17,7 +17,6 @@ classifiers = [
     "Intended Audience :: Developers",
     "License :: OSI Approved :: MIT License",
     "Programming Language :: Python :: 3",
-    "Programming Language :: Python :: 3.8",
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
@@ -49,7 +48,7 @@ dev = [
 
 [tool.ruff]
 line-length = 100
-target-version = "py38"
+target-version = "py39"
 exclude = [
     "generated",
     "copilot/generated",