Allow access to __version__ with imports

We move the top level imports into the 'setup' function in order to
allow setup.py to access the __version__ variable in the module without
having to have sphinx available.

I'm not sure if this is for the best but the ReadTheDocs build is
failing and it seems potentially related to this though I've no idea why
it is suddenly happening as we've not changed anything around this to my
understanding.

breathe/__init__.py

@@ -1,13 +1,16 @@
-from breathe.directives.setup import setup as directive_setup
-from breathe.file_state_cache import setup as file_state_cache_setup
-from breathe.renderer.sphinxrenderer import setup as renderer_setup
+__version__ = "4.31.0"
 
-from sphinx.application import Sphinx
 
-__version__ = "4.31.0"
+def setup(app):
+
+    from breathe.directives.setup import setup as directive_setup
+    from breathe.file_state_cache import setup as file_state_cache_setup
+    from breathe.renderer.sphinxrenderer import setup as renderer_setup
+
+    from sphinx.application import Sphinx
 
+    app: Sphinx = app
 
-def setup(app: Sphinx):
     directive_setup(app)
     file_state_cache_setup(app)
     renderer_setup(app)