packaging: stop exact-pinning build backends

Exact setuptools/wheel pins block platform-specific backend patches and
make isolated pip installs fail on Android/Termux Python. Keep a setuptools
lower bound only, drop the unused wheel backend pin, keep requirements.txt
aligned with core package deps, and install the Spaces extra in Docker
instead of pulling bitsandbytes by default.

Android/Termux is still not a supported runtime; the new platform note
documents host wheels, ANDROID_API_LEVEL, and --no-build-isolation so
pip does not isolate a second setuptools or compile manylinux-only
extensions.

README.md

@@ -170,7 +170,9 @@ and rollback requirements.
 Install `.[spaces,quantization]` instead when the UI must load supported
 bitsandbytes 8-bit or 4-bit models. Jetson users must follow the dedicated
 [Jetson bootstrap](docs/platforms/jetson.md); its bitsandbytes path is not yet
-supported.
+supported. Android/Termux is not a supported runtime; if you install there
+anyway, follow the [Android install notes](docs/platforms/android.md) so pip
+does not isolate a second setuptools or replace host wheels.
 
 ### 3. Google Colab (free GPU)