Fix Python file URL construction on Windows

...after d7628186 "Create temp copies of test
docs in Python/UITests"

Change-Id: I4b6a5149c68f9e6cdc0a2037c07b50e8acc3623f
Reviewed-on: https://gerrit.libreoffice.org/52486Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

unotest/source/python/org/libreoffice/unotest.py

@@ -9,6 +9,7 @@
 
 from __future__ import print_function
 
+import pathlib
 import subprocess
 import time
 import uuid
@@ -217,11 +218,7 @@ class UnoInProcess:
         return self.openDocFromAbsolutePath(path, asTemplate)
 
     def openDocFromAbsolutePath(self, file, asTemplate = False):
-        if os.name == "nt":
-            url = "file:///" + file
-        else:
-            url = "file://" + file
-        return self.openDocFromURL(url, asTemplate)
+        return self.openDocFromURL(pathlib.Path(file).as_uri(), asTemplate)
 
     def openDocFromURL(self, url, asTemplate = False):
         props = [("Hidden", True), ("ReadOnly", False), ("AsTemplate", asTemplate)]