bpo-30726: Fix elementtree warnings on Windows due to expat upgrade (#2319)

* bpo-30726: Fix elementtree warnings on Windows

Caused by usage of `getenv` which should be safe. And a few integer
truncations which should also be ok.

* bpo-30726: Don't ignore libexpat warnings which haypo intends to fix upstream

PCbuild/_elementtree.vcxproj

@@ -62,7 +62,7 @@
   <ItemDefinitionGroup>
     <ClCompile>
       <AdditionalIncludeDirectories>..\Modules\expat;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
-      <PreprocessorDefinitions>USE_PYEXPAT_CAPI;XML_STATIC;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>_CRT_SECURE_NO_WARNINGS;USE_PYEXPAT_CAPI;XML_STATIC;%(PreprocessorDefinitions)</PreprocessorDefinitions>
     </ClCompile>
     <Link>
       <BaseAddress>0x1D100000</BaseAddress>