fix: Use Union in type hinting

Use Union to be compatible with 3.7

Author: Cedric Khin

google/cloud/firestore_v1/collection.py

@@ -25,7 +25,7 @@
 from google.cloud.firestore_v1 import aggregation
 from google.cloud.firestore_v1.watch import Watch
 from google.cloud.firestore_v1 import document
-from typing import Any, Callable, Generator, Tuple
+from typing import Any, Callable, Generator, Tuple, Union
 
 # Types needed only for Type Hints
 from google.cloud.firestore_v1.transaction import Transaction
@@ -79,9 +79,9 @@ def _aggregation_query(self) -> aggregation.AggregationQuery:
  def add(
  self,
  document_data: dict,
- document_id: str | None = None,
+ document_id: Union[str, None] = None,
  retry: retries.Retry = gapic_v1.method.DEFAULT,
- timeout: float | None = None,
+ timeout: Union[float, None] = None,
  ) -> Tuple[Any, Any]:
  """Create a document in the Firestore database with the provided data.
 
@@ -121,9 +121,9 @@ def add(
 
  def list_documents(
  self,
- page_size: int | None = None,
+ page_size: Union[int, None] = None,
  retry: retries.Retry = gapic_v1.method.DEFAULT,
- timeout: float | None = None,
+ timeout: Union[float, None] = None,
  ) -> Generator[Any, Any, None]:
  """List all subdocuments of the current collection.
 
@@ -156,9 +156,9 @@ def _chunkify(self, chunk_size: int):
 
  def get(
  self,
- transaction: Transaction | None = None,
+ transaction: Union[Transaction, None] = None,
  retry: retries.Retry = gapic_v1.method.DEFAULT,
- timeout: float | None = None,
+ timeout: Union[float, None] = None,
  ) -> list:
  """Read the documents in this collection.
 
@@ -187,9 +187,9 @@ def get(
 
  def stream(
  self,
- transaction: Transaction | None = None,
+ transaction: Union[Transaction, None] = None,
  retry: retries.Retry = gapic_v1.method.DEFAULT,
- timeout: float | None = None,
+ timeout: Union[float, None] = None,
  ) -> Generator[document.DocumentSnapshot, Any, None]:
  """Read the documents in this collection.