Add fix for bug in Windows Python 3.6/3.7

TypeError: argument of type 'WindowsPath' is not iterable

docs/api.rst

@@ -56,8 +56,8 @@ OCRmyPDF does not.
     On Windows, the script that calls ``ocrmypdf.ocr()`` must be protected
     by an "ifmain" guard (``if __name__ == '__main__'``) or you must use
     ``ocrmypdf.ocr(...use_threads=True)``. If you do not take at least one
-    of these steps, Windows fork semantics will prevent OCRmyPDF from working
-    correct.
+    of these steps, Windows process semantics will prevent OCRmyPDF from working
+    correctly.
 
 Logging
 -------

src/ocrmypdf/exec/__init__.py

@@ -94,6 +94,11 @@ def run(args, *, env=None, **kwargs):
 def fix_windows_args(program, args, env):
     """Adjust our desired program and command line arguments for use on Windows"""
 
+    if sys.version_info < (3, 8):
+        # bpo-33617 - Windows needs manual Path -> str conversion
+        args = [os.fspath(arg) for arg in args]
+        program = os.fspath(program)
+
     # If we are running a .py on Windows, ensure we call it with this Python
     # (to support test suite shims)
     if program.lower().endswith('.py'):