[Fix] Use correct optional typing in WorkspaceClient for mypy (#760)

Without this change, the SDK is not very `mypy`-compatible. This PR
fixes `arg-type` check:

```
cmd [3] | mypy --disable-error-code 'annotation-unchecked' --exclude 'tests/resources/*' --exclude dist .
error: Argument "auth_type" to "WorkspaceClient" has incompatible type "str | None"; expected "str"  [arg-type]
error: Argument "token" to "WorkspaceClient" has incompatible type "str | None"; expected "str"  [arg-type]
```

`Optional[X]` is py3.8 and py3.9 way of expressing optional types, but
in py3.10+ it gets transformed into a union type of `X | None`, which is
not supported by py3.9. py3.9 EOL is 31 Oct 2025, so we have to deal
with it somehow until then.

Author: nfx

.codegen/__init__.py.tmpl

@@ -41,14 +41,14 @@ class WorkspaceClient:
     """
     The WorkspaceClient is a client for the workspace-level Databricks REST API.
     """
-    def __init__(self, *{{range $args}}, {{.}}: str = None{{end}},
-                 debug_truncate_bytes: int = None,
-                 debug_headers: bool = None,
+    def __init__(self, *{{range $args}}, {{.}}: Optional[str] = None{{end}},
+                 debug_truncate_bytes: Optional[int] = None,
+                 debug_headers: Optional[bool] = None,
                  product="unknown",
                  product_version="0.0.0",
-                 credentials_strategy: CredentialsStrategy = None,
-                 credentials_provider: CredentialsStrategy = None,
-                 config: client.Config = None):
+                 credentials_strategy: Optional[CredentialsStrategy] = None,
+                 credentials_provider: Optional[CredentialsStrategy] = None,
+                 config: Optional[client.Config] = None):
         if not config:
           config = client.Config({{range $args}}{{.}}={{.}}, {{end}}
             credentials_strategy=credentials_strategy,
@@ -110,14 +110,14 @@ class AccountClient:
     The AccountClient is a client for the account-level Databricks REST API.
     """
 
-    def __init__(self, *{{range $args}}, {{.}}: str = None{{end}},
-                 debug_truncate_bytes: int = None,
-                 debug_headers: bool = None,
+    def __init__(self, *{{range $args}}, {{.}}: Optional[str] = None{{end}},
+                 debug_truncate_bytes: Optional[int] = None,
+                 debug_headers: Optional[bool] = None,
                  product="unknown",
                  product_version="0.0.0",
-                 credentials_strategy: CredentialsStrategy = None,
-                 credentials_provider: CredentialsStrategy = None,
-                 config: client.Config = None):
+                 credentials_strategy: Optional[CredentialsStrategy] = None,
+                 credentials_provider: Optional[CredentialsStrategy] = None,
+                 config: Optional[client.Config] = None):
         if not config:
           config = client.Config({{range $args}}{{.}}={{.}}, {{end}}
             credentials_strategy=credentials_strategy,

databricks/sdk/__init__.py

@@ -92,11 +92,11 @@ class Config:
     def __init__(self,
                  *,
                  # Deprecated. Use credentials_strategy instead.
-                 credentials_provider: CredentialsStrategy = None,
-                 credentials_strategy: CredentialsStrategy = None,
+                 credentials_provider: Optional[CredentialsStrategy] = None,
+                 credentials_strategy: Optional[CredentialsStrategy] = None,
                  product=None,
                  product_version=None,
-                 clock: Clock = None,
+                 clock: Optional[Clock] = None,
                  **kwargs):
         self._header_factory = None
         self._inner = {}