Add mypy configuration for sshsig library

Configure mypy to ignore missing type stubs for the sshsig library,
similar to how we handle other optional dependencies like fastimport
and patiencediff.

pyproject.toml

@@ -101,6 +101,12 @@ ignore_missing_imports = true
 disallow_untyped_calls = false
 warn_return_any = false
 
+[[tool.mypy.overrides]]
+module = "sshsig.*"
+ignore_missing_imports = true
+disallow_untyped_calls = false
+warn_return_any = false
+
 [tool.setuptools]
 packages = [
     "dulwich",