bpo-35081: Move accu.h to Include/internal/pycore_accu.h (GH-10271)

The accu.h header is no longer part of the Python C API: it has been
moved to the "internal" headers which are restricted to Python
itself.

Replace #include "accu.h" with #include "pycore_accu.h".
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index ebc35a7..6becb8a 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -79,7 +79,6 @@
   </ItemDefinitionGroup>
   <ItemGroup>
     <ClInclude Include="..\Include\abstract.h" />
-    <ClInclude Include="..\Include\accu.h" />
     <ClInclude Include="..\Include\asdl.h" />
     <ClInclude Include="..\Include\ast.h" />
     <ClInclude Include="..\Include\bitset.h" />
@@ -112,6 +111,7 @@
     <ClInclude Include="..\Include\graminit.h" />
     <ClInclude Include="..\Include\grammar.h" />
     <ClInclude Include="..\Include\import.h" />
+    <ClInclude Include="..\Include\internal\pycore_accu.h" />
     <ClInclude Include="..\Include\internal\pycore_atomic.h" />
     <ClInclude Include="..\Include\internal\pycore_ceval.h" />
     <ClInclude Include="..\Include\internal\pycore_condvar.h" />
@@ -154,7 +154,6 @@
     <ClInclude Include="..\Include\pyerrors.h" />
     <ClInclude Include="..\Include\pyexpat.h" />
     <ClInclude Include="..\Include\pyfpe.h" />
-    <ClInclude Include="..\Include\internal\pygetopt.h" />
     <ClInclude Include="..\Include\pylifecycle.h" />
     <ClInclude Include="..\Include\pymath.h" />
     <ClInclude Include="..\Include\pytime.h" />