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

(cherry picked from commit 04856c2) Co-authored-by: Paul Monson <paulmon@users.noreply.github.com>

Author: miss-islington

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)">