RFC: Adding new variants to select the desired Qt binding

This adds:

- `mne[full-no-qt]`: no Qt bindings
- `mne[full-pyqt6]`: equivalent to `mne[full]`, Qt6 bindings
- `mne[full-pyside6]`: PySide6 bindings

Fixes #12626

pyproject.toml

@@ -59,16 +59,18 @@ data = []
 hdf5 = ["h5io", "pymatreader"]
 
 # Dependencies for full MNE-Python functionality (other than raw/epochs export)
-full = [
+# We first define a variant without any Qt bindings. The "complete" variant, mne[full],
+# makes an opinionated choice and installs PyQt6.
+# We also offter two more variants: mne[full-qt6] (which is equivalent to mne[full]),
+# and mne[full-pyside6], which will install PySide6 instead of PyQt6.
+full-no-qt = [
     "mne[hdf5]",
     "qtpy",
-    "PyQt6!=6.6.0",
-    "PyQt6-Qt6!=6.6.0,!=6.7.0",
     "pyobjc-framework-Cocoa>=5.2.0; platform_system=='Darwin'",
     "sip",
     "scikit-learn",
     "nibabel",
-    "openmeeg>=2.5.5",
+    # "openmeeg>=2.5.5",
     "numba",
     "h5py",
     "pandas",
@@ -106,6 +108,19 @@ full = [
     "defusedxml",
     "neo",
 ]
+full = [
+    "mne[full-no-qt]",
+    "PyQt6!=6.6.0",
+    "PyQt6-Qt6!=6.6.0,!=6.7.0",
+]
+full-pyqt6 = [
+    "mne[full]"
+]
+full-pyside6 = [
+    "mne[full-no-qt]",
+    "PySide6"
+]
+
 
 # Dependencies for running the test infrastructure
 test = [