Adds support for building with VisualStudioVersion = 18.0 (GH-138980)

(cherry picked from commit 6920fa4)

Co-authored-by: Steve Dower <steve.dower@python.org>
Co-authored-by: Petr Viktorin <encukou@gmail.com>

Author: 3 people

PCbuild/python.props

@@ -11,6 +11,7 @@
 
     We set BasePlatformToolset for ICC's benefit, it's otherwise ignored.
     -->
+    <BasePlatformToolset Condition="'$(BasePlatformToolset)' == '' and '$(VisualStudioVersion)' == '18.0'">v143</BasePlatformToolset>
     <BasePlatformToolset Condition="'$(BasePlatformToolset)' == '' and '$(VisualStudioVersion)' == '17.0'">v143</BasePlatformToolset>
     <BasePlatformToolset Condition="'$(BasePlatformToolset)' == '' and '$(VisualStudioVersion)' == '16.0'">v142</BasePlatformToolset>
     <BasePlatformToolset Condition="'$(BasePlatformToolset)' == '' and ('$(MSBuildToolsVersion)' == '15.0' or '$(VisualStudioVersion)' == '15.0')">v141</BasePlatformToolset>