restore python 3.6 support in check_qml_module.py

Leap 15 comes with python 3.6 by default. Since a few months check_qml_module.py used language features, which are only available in 3.7 or later. Since that time I use a change like this in a few places in that file, to be able to compile and run the master branch:

--- a/buildsystem/check_qml_module.py
+++ b/buildsystem/check_qml_module.py
@@ -50,7 +50,7 @@ class QmlModuleChecker:
                 "-qmlFiles", f.name,
                 "-importPath", self.qmlpath
             ],
-            capture_output=True,
+            stdout=subprocess.PIPE, stderr=subprocess.PIPE,
             )

         if ret.returncode != 0:

There are no obvious failures after this change AFAICS.

Please consider removal of the capture_output parameter, in favor of the stdout/stderr parameters.

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