chore: remove Python < 3.9 logic and update dependencies (temporalio#728)

- Bump the minimum supported Python version from 3.8 to 3.9
- Remove macOS-specific code for Python < 3.8
- Update references in README, build.py, and poetry.lock accordingly

Closes temporalio#728

Author: ozeranskii

README.md

@@ -1358,7 +1358,7 @@ users are encouraged to not use gevent in asyncio applications (including Tempor
 
 # Development
 
-The Python SDK is built to work with Python 3.8 and newer. It is built using
+The Python SDK is built to work with Python 3.9 and newer. It is built using
 [SDK Core](https://github.com/temporalio/sdk-core/) which is written in Rust.
 
 ### Building

build.py

@@ -16,7 +16,7 @@ def build(setup_kwargs):
                 path="temporalio/bridge/Cargo.toml",
                 binding=Binding.PyO3,
                 py_limited_api=True,
-                features=["pyo3/abi3-py38"],
+                features=["pyo3/abi3-py39"],
             )
         ],
         zip_safe=False,