HTML Templates: Expose full sphinx version tuple

Previously, there was only a string which is problematic
when you want to programmatically compare versions
for compatibility reasons.

doc/templating.rst

@@ -371,7 +371,16 @@ in the future.
 
 .. data:: sphinx_version
 
-   The version of Sphinx used to build.
+   The version of Sphinx used to build represented as a string for example "3.5.1".
+
+.. data:: sphinx_version_tuple
+
+   The version of Sphinx used to build represented as a tuple of five elements.
+   For Sphinx version 3.5.1 beta 3 this would be `(3, 5, 1, 'beta', 3)``.
+   The fourth element can be one of: ``alpha``, ``beta``, ``rc``, ``final``.
+   ``final`` always has 0 as the last element.
+
+   .. versionadded:: 4.2
 
 .. data:: style
 

sphinx/builders/html/__init__.py

@@ -26,6 +26,7 @@
 from docutils.utils import relative_path
 
 from sphinx import __display_version__, package_dir
+from sphinx import version_info as sphinx_version
 from sphinx.application import Sphinx
 from sphinx.builders import Builder
 from sphinx.config import ENUM, Config
@@ -508,7 +509,8 @@ def prepare_writing(self, docnames: Set[str]) -> None:
             'script_files': self.script_files,
             'language': self.config.language,
             'css_files': self.css_files,
-            'sphinx_version': __display_version__,
+            'sphinx_version': sphinx_version,
+            'sphinx_version_tuple': sphinx_version,
             'style': self._get_style_filename(),
             'rellinks': rellinks,
             'builder': self.name,