Start installing PyQt6 into the Linux ARM64 venv by default

Now that an ARM wheel is on PyPI, we no longer need to rely on a
system PyQt to build on ARM. The install is skipped when PYTHONPATH
is set, so older distros with glibc <2.39 can continue to use the
system packages instead.

Author: dae

build/configure/src/python.rs

@@ -28,8 +28,8 @@ pub fn setup_venv(build: &mut Build) -> Result<()> {
         ]
     } else if cfg!(target_os = "macos") {
         inputs!["python/requirements.qt6_mac.txt",]
-    } else if cfg!(all(target_os = "linux", target_arch = "aarch64")) {
-        // system-provided Qt on ARM64
+    } else if std::env::var("PYTHONPATH").is_ok() {
+        // assume we have a system-provided Qt
         inputs![]
     } else {
         // normal linux

docs/linux.md

@@ -52,13 +52,16 @@ To play and record audio during development, install mpv and lame.
 
 ## ARM64 support
 
-Other platforms download PyQt binary wheels from PyPI. There are no PyQt wheels available
-for ARM Linux, so you will need to rely on your system-provided libraries instead. Your distro
-will need to have Python 3.9 or later.
+If you're on a glibc 2.39+ system, you can skip the rest of this section.
 
-After installing the system libraries (eg 'sudo apt install python3-pyqt6.qt{quick,webengine} python3-venv'),
+If your system has an older glibc, you won't be able to use the PyQt wheels that are
+available in pip/PyPy, and will need to use your system-installed PyQt instead.
+Your distro will also need to have Python 3.9 or later.
+
+After installing the system libraries (eg:
+'sudo apt install python3-pyqt6.qt{quick,webengine} python3-venv pyqt6-dev-tools'),
 find the place they are installed (eg '/usr/lib/python3/dist-packages'). On modern Ubuntu, you'll
-need 'sudo apt remove python3-protobuf'. Then before running any commands like './run', tell Anki where
+also need 'sudo apt remove python3-protobuf'. Then before running any commands like './run', tell Anki where
 the packages can be found:
 
 ```