windows: update search string in OpenSSL project file

The exact string changed in CPython 3.7.3, which we now build.

Author: indygreg

cpython-windows/build.py

@@ -509,7 +509,7 @@ def copy_link_to_lib(p: pathlib.Path):
     <Copy SourceFiles="@(_SSLDLL)" DestinationFolder="$(OutDir)" />
   </Target>
   <Target Name="_CleanSSLDLL" BeforeTargets="Clean">
-    <Delete Files="@(_SSLDLL->'$(OutDir)%(Filename)%(Extension)')" />
+    <Delete Files="@(_SSLDLL->'$(OutDir)%(Filename)%(Extension)')" TreatErrorsAsWarnings="true" />
   </Target>
 '''