chore: mypy mini cleanups

Note regarding the `ignore`d super() call: mypy thinks that the
`super()` call in `ManabiS3Provider.__init__()` goes to the direct base
class `ManabiProvider` and is thus missing the required `root_folder`
parameter, but this ain't so: This calls `DAVProvider.__init__()` which
doesn't take that param.

Then we add a bit of typing sugar and make mypy shut up about missing
type information from 3rd party imports.

A few smaller issues that mypy finds in CI but not locally are addressed
as well.

manabi/filesystem.py

@@ -227,7 +227,7 @@ def get_content(self):
             transport_params={"client": self.s3},
         )
 
-    def begin_write(self, *, content_type):
+    def begin_write(self, *, content_type=None):
         """
         Open content as a stream for writing.
 
@@ -257,7 +257,7 @@ def __init__(
         shadow=None,
         cb_hook_config: Optional[CallbackHookConfig] = None,
     ):
-        super(FilesystemProvider, self).__init__()
+        super(FilesystemProvider, self).__init__()  # type: ignore
 
         if not root_folder:
             raise ValueError("Invalid root path: {dir}".format(dir=root_folder))
@@ -269,7 +269,7 @@ def __init__(
         else:
             self.shadow = {}
 
-        self.fs_opts = {}
+        self.fs_opts: Dict[str, Any] = {}
         # Get shadow map and convert keys to lower case
         self.shadow_map = self.fs_opts.get("shadow_map") or {}
         if self.shadow_map:

pyproject.toml

@@ -94,6 +94,8 @@ commit_author = "github-actions <[email protected]>"
 [tool.mypy]
 check_untyped_defs = true
 
+ignore_missing_imports = true
+
 [tool.flake8]
 ignore = """
     E501,