Enforce Python to run in UTF-8 mode

When running Python via QProcess on Windows, the output code page will be set to the system default instead of UTF-8.
This commit enforces that UTF-8 will be used unconditionally.

Supersedes #23629.
PR #23633.

src/base/utils/foreignapps.h

@@ -38,6 +38,7 @@
 namespace Utils::ForeignApps
 {
     inline const QString PYTHON_ISOLATE_MODE_FLAG = u"-I"_s;
+    inline const QString PYTHON_UTF8_MODE_FLAG = u"-Xutf8=1"_s;
 
     struct PythonInfo
     {