bpo-33673: Install python-docs-theme even if Sphinx is already installed (GH-7163)

Doc/make.bat

@@ -13,7 +13,13 @@ if not defined SPHINXBUILD (
     %PYTHON% -c "import sphinx" > nul 2> nul
     if errorlevel 1 (
         echo Installing sphinx with %PYTHON%
-        %PYTHON% -m pip install sphinx python-docs-theme
+        %PYTHON% -m pip install sphinx
+        if errorlevel 1 exit /B
+    )
+    %PYTHON% -c "import python_docs_theme" > nul 2> nul
+    if errorlevel 1 (
+        echo Installing python-docs-theme with %PYTHON%
+        %PYTHON% -m pip install python-docs-theme
         if errorlevel 1 exit /B
     )
     set SPHINXBUILD=%PYTHON% -c "import sphinx, sys; sys.argv[0] = 'sphinx-build'; sphinx.main()"