v1.7.5: fix avvio Windows ("Failed to resolve Python.Runtime.Loader")

PyInstaller non bundlava i file di supporto di pythonnet/clr_loader
(Python.Runtime.dll, nethost.dll, runtimeconfig.json), per cui al
caricamento di webview.platforms.winforms.py l'import 'clr' moriva.

Aggiunti --collect-all pythonnet/clr_loader + copy-metadata.
Pinnata pythonnet a 3.0.3 (l'ultima compatibile con PyInstaller
senza patch aggiuntive).

requirements.txt

@@ -5,4 +5,4 @@ mutagen>=1.47.0
 pyobjc-core>=10.0 ; sys_platform == "darwin"
 pyobjc-framework-Cocoa>=10.0 ; sys_platform == "darwin"
 pyobjc-framework-WebKit>=10.0 ; sys_platform == "darwin"
-pythonnet>=3.0 ; sys_platform == "win32"
+pythonnet==3.0.3 ; sys_platform == "win32"