builder: use pathlib for internal imgmath override

Updating the internal overwrite of the imgmath temporary directory to
use the pathlib module. Upstream version of Sphinx has updated its
implementation to use pathlib, and using a str causes issues when
running the imgmath extension.

Using `Path` should be fine for existing/older versions of Sphinx, as
its value used with `os.path.join` handles `Path` objects.

Signed-off-by: James Knight <git@jdknight.me>

sphinxcontrib/confluencebuilder/builder.py

@@ -131,8 +131,8 @@ def init(self):
         # The imgmath extension allows a builder to override where temporary
         # files are build -- use this to hint to using a temporary directory
         # on the same partition the output directory to help prevent issues.
-        self._imgmath_tempdir = tempfile.mkdtemp(
-            prefix='.imgmath-', dir=self.out_dir)
+        self._imgmath_tempdir = Path(tempfile.mkdtemp(
+            prefix='.imgmath-', dir=self.out_dir))
 
         if self.config.confluence_publish:
             process_ask_configs(self.config)