Try to fix french PDF generation, see python/python-docs-fr#199

Author: JulienPalard

build_docs.py

@@ -62,7 +62,8 @@
            '-D latex_elements.fontenc='],
     'fr': ['-D latex_engine=xelatex',
            '-D latex_elements.inputenc=',
-           '-D latex_elements.fontenc='],
+           '-D latex_elements.fontenc=',
+           '-D latex_elements.babel=\\\\usepackage{babel}'],
     'en': ['-D latex_engine=xelatex',
            '-D latex_elements.inputenc=',
            '-D latex_elements.fontenc=']