Fix compability with Python3.9 (#16)

* Update python-app.yml

* Update backed_df.py

* Update requirements.txt

* Update requirements.txt

.github/workflows/python-app.yml

@@ -19,10 +19,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
-    - name: Set up Python 3.10
+    - name: Set up Python 3.9
       uses: actions/setup-python@v3
       with:
-        python-version: "3.10"
+        python-version: "3.9"
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip

cap_anndata/backed_df.py

@@ -1,6 +1,6 @@
 import pandas as pd
 import numpy as np
-from typing import List, Any
+from typing import List, Any, Union
 import logging
 
 from pandas._typing import Self
@@ -65,5 +65,5 @@ def merge(self, right, **kwargs) -> Self:
         column_order = np.append(self.column_order, new_columns)
         return self.from_df(result, column_order=column_order)
 
-    def copy(self, deep: bool_t | None = True) -> Self:
+    def copy(self, deep: Union[bool_t, None] = True) -> Self:
         return self.from_df(super().copy(deep=deep), column_order=self.column_order)