fix: convert path separators to match os.sep

this is a focused fix for the problem referenced in:

pypa#445 (comment)

Author: mawillcockson

flit/sdist.py

@@ -96,6 +96,7 @@ def find_nearest_pkg(rel_path):
 
 
 def include_path(p):
+    p = str(Path(p))
     return not (p.startswith('dist' + os.sep)
                 or (os.sep+'__pycache__' in p)
                 or p.endswith('.pyc'))