Fix strings containing deprecated illegal escape sequences (#648)

Python 3.6 and above has deprecated illegal string escape sequences. Escape sequences are preceded by a '\' and valid ones are "\n\t\r" etc. Illegal ones are not in the list https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals.

Previous to Python 3.6 these illegal sequences were ignored and the '\' was left in place. Pylint has been noting these for a while now.

This PR corrects these sequences in Gramps. I used

find . -name "*.py" | xargs -t -n 1 python3 -Wd -m py_compile 2>&1 | grep Depre

to locate the failing strings.

windows/nonAIO/check_gtk_install.py

@@ -133,7 +133,8 @@ def ScanDependencyFileForErrors(fname):
             parts = line[pthend_idx:].split()
             OK = False
             if dirname.startswith(os.path.join(sysroot, 'winsxs').lower()) \
-            or dirname.startswith(os.path.join(sys.prefix, 'lib\site-packages\gtk-2.0').lower()):
+                or dirname.startswith(os.path.join(
+                    sys.prefix, r'lib\site-packages\gtk-2.0').lower()):
                 OK = True
 
             for pth in acceptablePaths: