bpo-37238: Enable building for Windows using Visual Studio 2019 (GH-13988)

PCbuild/python.props

@@ -10,6 +10,7 @@
 
     We set BasePlatformToolset for ICC's benefit, it's otherwise ignored.
     -->
+    <BasePlatformToolset Condition="'$(BasePlatformToolset)' == '' and ('$(MSBuildToolsVersion)' == '16.0' or '$(VisualStudioVersion)' == '16.0')">v142</BasePlatformToolset>
     <BasePlatformToolset Condition="'$(BasePlatformToolset)' == '' and ('$(MSBuildToolsVersion)' == '15.0' or '$(VisualStudioVersion)' == '15.0')">v141</BasePlatformToolset>
     <BasePlatformToolset Condition="'$(BasePlatformToolset)' == '' and '$(VCTargetsPath14)' != ''">v140</BasePlatformToolset>
     <BasePlatformToolset Condition="'$(BasePlatformToolset)' == '' and '$(VCTargetsPath12)' != ''">v120</BasePlatformToolset>

PCbuild/pythoncore.vcxproj

@@ -511,7 +511,7 @@
       </ClCompile>
     </ItemGroup>
   </Target>
-  <Target Name="_WarnAboutToolset" BeforeTargets="PrepareForBuild" Condition="$(PlatformToolset) != 'v140' and $(PlatformToolset) != 'v141'">
+  <Target Name="_WarnAboutToolset" BeforeTargets="PrepareForBuild" Condition="$(PlatformToolset) != 'v140' and $(PlatformToolset) != 'v141' and $(PlatformToolset) != 'v142'">
     <Warning Text="Toolset $(PlatformToolset) is not used for official builds. Your build may have errors or incompatibilities." />
   </Target>
   <Target Name="_WarnAboutZlib" BeforeTargets="PrepareForBuild" Condition="!$(IncludeExternals)">