Make QtWebEngineWidgets import optional.

Upstream imports qtpy.QtWebEngineWidgets unconditionally before
QCoreApplication construction to work around QTBUG-46720 (the binding
needs to be loaded before the Qt app is up, or off-screen rendering
breaks). The module is only actually used by the EELS-database plugin
(hyperspyui/plugins/eelsdb.py).

::gentoo's dev-python/qtpy[webengine] route works, but pulls
pyqt6-webengine + qtwebengine — substantial chromium-derived weight
for users who don't need EELSDB lookups. PluginManager.discover()
already catches arbitrary exceptions from plugin imports, so wrapping
the eager import in try/except lets EELSDB self-disable gracefully
when WebEngine isn't installed; users who want it can simply install
dev-python/pyqt6-webengine and restart.

The same pattern is applied to tests/conftest.py for consistency,
though the test path isn't exercised by the Gentoo ebuild.

--- a/hyperspyui/__main__.py
+++ b/hyperspyui/__main__.py
@@ -115,7 +115,12 @@
     # QtWebEngineWidgets must be imported before a QCoreApplication instance
     # is created (used in eelsdb plugin)
     # Avoid a bug in Qt: https://bugreports.qt.io/browse/QTBUG-46720
-    from qtpy import QtWebEngineWidgets  # noqa: F401
+    # Gentoo: WebEngine is optional - the eelsdb plugin self-disables
+    # via PluginManager.discover()'s exception handler when absent.
+    try:
+        from qtpy import QtWebEngineWidgets  # noqa: F401
+    except ImportError:
+        pass

     # Need to set early to make QSettings accessible
     QCoreApplication.setApplicationName("HyperSpyUI")
--- a/hyperspyui/tests/conftest.py
+++ b/hyperspyui/tests/conftest.py
@@ -12,7 +12,10 @@
 # QtWebEngineWidgets must be imported before a QCoreApplication instance
 # is created (used in eelsdb plugin)
 # Avoid a bug in Qt: https://bugreports.qt.io/browse/QTBUG-46720
-from qtpy import QtWebEngineWidgets  # noqa: F401
+try:
+    from qtpy import QtWebEngineWidgets  # noqa: F401
+except ImportError:
+    pass

 QCoreApplication = QtCore.QCoreApplication
 QSettings = QtCore.QSettings