fix(celery): canvas eta param to be nullable (#33)

Originally optional, at runtime it's just `kwargs`, so we're kind of
flying blind when typing it

related: #32

typings/celery/canvas.pyi

@@ -40,7 +40,7 @@ class Signature(Dict[str, Any]):
         shadow: Optional[str] = ...,
         # apply_async options
         countdown: float = ...,
-        eta: datetime = ...,
+        eta: datetime | None = ...,
         expires: Union[float, datetime] = ...,
         retry: bool = ...,
         retry_policy: Mapping[str, Any] = ...,
@@ -77,7 +77,7 @@ class Signature(Dict[str, Any]):
         shadow: Optional[str] = ...,
         # apply_async options
         countdown: float = ...,
-        eta: datetime = ...,
+        eta: datetime | None = ...,
         expires: Union[float, datetime] = ...,
         retry: bool = ...,
         retry_policy: Mapping[str, Any] = ...,
@@ -103,7 +103,7 @@ class Signature(Dict[str, Any]):
         shadow: Optional[str] = ...,
         # apply_async options
         countdown: float = ...,
-        eta: datetime = ...,
+        eta: datetime | None = ...,
         expires: Union[float, datetime] = ...,
         retry: bool = ...,
         retry_policy: Mapping[str, Any] = ...,
@@ -143,7 +143,7 @@ class Signature(Dict[str, Any]):
         shadow: Optional[str] = ...,
         # apply_async options
         countdown: float = ...,
-        eta: datetime = ...,
+        eta: datetime | None = ...,
         expires: Union[float, datetime] = ...,
         retry: bool = ...,
         retry_policy: Mapping[str, Any] = ...,
@@ -201,7 +201,7 @@ class _chain(Signature):
         link_error: Optional[Signature] = ...,
         shadow: Optional[str] = ...,
         countdown: float = ...,
-        eta: datetime = ...,
+        eta: datetime | None = ...,
         expires: Union[float, datetime] = ...,
         retry: bool = ...,
         retry_policy: Mapping[str, Any] = ...,
@@ -237,7 +237,7 @@ class _basemap(Signature):
         link_error: Optional[Signature] = ...,
         shadow: Optional[str] = ...,
         countdown: float = ...,
-        eta: datetime = ...,
+        eta: datetime | None = ...,
         expires: Union[float, datetime] = ...,
         retry: bool = ...,
         retry_policy: Mapping[str, Any] = ...,
@@ -275,7 +275,7 @@ class chunks(Signature):
         link_error: Optional[Signature] = ...,
         shadow: Optional[str] = ...,
         countdown: float = ...,
-        eta: datetime = ...,
+        eta: datetime | None = ...,
         expires: Union[float, datetime] = ...,
         retry: bool = ...,
         retry_policy: Mapping[str, Any] = ...,
@@ -309,7 +309,7 @@ class group(Signature):
         link_error: Optional[Signature] = ...,
         shadow: Optional[str] = ...,
         countdown: float = ...,
-        eta: datetime = ...,
+        eta: datetime | None = ...,
         expires: Union[float, datetime] = ...,
         retry: bool = ...,
         retry_policy: Mapping[str, Any] = ...,
@@ -350,7 +350,7 @@ class chord(Signature):
         link_error: Optional[Signature] = ...,
         shadow: Optional[str] = ...,
         countdown: float = ...,
-        eta: datetime = ...,
+        eta: datetime | None = ...,
         expires: Union[float, datetime] = ...,
         retry: bool = ...,
         retry_policy: Mapping[str, Any] = ...,