chore: remove stale code for conditional import of Literal (#2676)

# Description
The codebase contains the following code twice:

```
if sys.version_info >= (3, 8):
    from typing import Literal
else:
    from typing_extensions import Literal
```

I believe this can be removed, since
[pyproject.toml](https://github.com/delta-io/delta-rs/blob/f432c4f8337c2b0d47958645684e5df336c61522/python/pyproject.toml#L10)
specifies that the minimum Python version for the project is 3.8:

```toml
requires-python = ">=3.8"
```

# Related Issue(s)
<!---
For example:

- closes #106
--->

# Documentation

<!---
Share links to useful documentation
--->

python/deltalake/_internal.pyi

@@ -1,10 +1,4 @@
-import sys
-from typing import Any, Dict, List, Mapping, Optional, Tuple, Union
-
-if sys.version_info >= (3, 8):
-    from typing import Literal
-else:
-    from typing_extensions import Literal
+from typing import Any, Dict, List, Literal, Mapping, Optional, Tuple, Union
 
 import pyarrow
 import pyarrow.fs as fs

python/deltalake/writer.py

@@ -1,5 +1,4 @@
 import json
-import sys
 import uuid
 from dataclasses import dataclass
 from datetime import date, datetime
@@ -13,6 +12,7 @@
     Iterable,
     Iterator,
     List,
+    Literal,
     Mapping,
     Optional,
     Protocol,
@@ -22,21 +22,14 @@
 )
 from urllib.parse import unquote
 
-from deltalake import Schema as DeltaSchema
-from deltalake.fs import DeltaStorageHandler
-
-from ._util import encode_partition_value
-
-if sys.version_info >= (3, 8):
-    from typing import Literal
-else:
-    from typing_extensions import Literal
-
 import pyarrow as pa
 import pyarrow.dataset as ds
 import pyarrow.fs as pa_fs
 from pyarrow import RecordBatchReader
 
+from deltalake import Schema as DeltaSchema
+from deltalake.fs import DeltaStorageHandler
+
 from ._internal import DeltaDataChecker as _DeltaDataChecker
 from ._internal import batch_distinct
 from ._internal import convert_to_deltalake as _convert_to_deltalake
@@ -45,6 +38,7 @@
 )
 from ._internal import write_new_deltalake as write_deltalake_pyarrow
 from ._internal import write_to_deltalake as write_deltalake_rust
+from ._util import encode_partition_value
 from .exceptions import DeltaProtocolError, TableNotFoundError
 from .schema import (
     convert_pyarrow_dataset,