Fixes warnings when building python3.dll due to the .def file accumulating…

Fixes warnings when building python3.dll due to the .def file accumulating multiple copies of each line.
Adds shebang line to prepare_ssl so it will run with py.exe.

PCbuild/prepare_ssl.py

+#! /usr/bin/env python3
 # Script for preparing OpenSSL for building on Windows.
 # Uses Perl to create nmake makefiles and otherwise prepare the way
 # for building on 32 or 64 bit platforms.

PCbuild/python3dll.vcxproj

@@ -109,7 +109,7 @@
       </_Lines>
     </ItemGroup>
     <MakeDir Directories="$(IntDir)" />
-    <WriteLinesToFile File="$(IntDir)python3_d.def" Lines="@(_Lines->'%(New)')" />
+    <WriteLinesToFile File="$(IntDir)python3_d.def" Lines="@(_Lines->'%(New)')" Overwrite="true" />
   </Target>
   
   <Target Name="BuildStubDef" BeforeTargets="PreLinkEvent" Inputs="..\PC\python3.def" Outputs="$(IntDir)python3stub.def">
@@ -132,7 +132,7 @@
       <_Lines Include="@(_Symbols->'%(Symbol)')" />
     </ItemGroup>
     <MakeDir Directories="$(IntDir)" />
-    <WriteLinesToFile File="$(IntDir)python3stub.def" Lines="@(_Lines)" />
+    <WriteLinesToFile File="$(IntDir)python3stub.def" Lines="@(_Lines)" Overwrite="true" />
   </Target>
   <Target Name="_CleanStubDef" BeforeTargets="CoreClean">
     <ItemGroup>