Use LBYL for imports where EAFP is a mypy type error

The conditional imports of Literal from either typing or
typing_extensions have to be done with if-else on the Python
version rather than with try-except, or it is a static type error.

This makes that change, checking sys.version_info.

(See discussion in gitpython-developers#1861 and gitpython-developers#1862 for broader context and
background on why this logic, before and after this change, is
repeated across multiple modules.)

This also reorders/regroups imports for consistency in some places,
especially where a new import of the sys module (for version_info)
would otherwise exacerbate inconsistency.

Since the merge commit 0b99041, the number of mypy errors had
increased from 5 to 10. This fixes all the new mypy errors, so the
count is back to 5.

git/objects/blob.py

@@ -4,12 +4,13 @@
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
 from mimetypes import guess_type
-from . import base
+import sys
 
+from . import base
 
-try:
+if sys.version_info >= (3, 8):
     from typing import Literal
-except ImportError:
+else:
     from typing_extensions import Literal
 
 __all__ = ("Blob",)

git/objects/commit.py

@@ -3,57 +3,57 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
+from collections import defaultdict
 import datetime
+from io import BytesIO
+import logging
+import os
 import re
 from subprocess import Popen, PIPE
+import sys
+from time import altzone, daylight, localtime, time, timezone
+
 from gitdb import IStream
-from git.util import hex_to_bin, Actor, Stats, finalize_process
-from git.diff import Diffable
 from git.cmd import Git
+from git.diff import Diffable
+from git.util import hex_to_bin, Actor, Stats, finalize_process
 
 from .tree import Tree
-from . import base
 from .util import (
     Serializable,
     TraversableIterableObj,
-    parse_date,
     altz_to_utctz_str,
-    parse_actor_and_date,
     from_timestamp,
+    parse_actor_and_date,
+    parse_date,
 )
-
-from time import time, daylight, altzone, timezone, localtime
-import os
-from io import BytesIO
-import logging
-from collections import defaultdict
-
+from . import base
 
 # typing ------------------------------------------------------------------
 
 from typing import (
     Any,
+    Dict,
     IO,
     Iterator,
     List,
     Sequence,
     Tuple,
-    Union,
     TYPE_CHECKING,
+    Union,
     cast,
-    Dict,
 )
 
-from git.types import PathLike
-
-try:
+if sys.version_info >= (3, 8):
     from typing import Literal
-except ImportError:
+else:
     from typing_extensions import Literal
 
+from git.types import PathLike
+
 if TYPE_CHECKING:
-    from git.repo import Repo
     from git.refs import SymbolicReference
+    from git.repo import Repo
 
 # ------------------------------------------------------------------------
 

git/objects/submodule/base.py

@@ -39,7 +39,6 @@
     sm_section,
 )
 
-
 # typing ----------------------------------------------------------------------
 
 from typing import (
@@ -54,13 +53,13 @@
     cast,
 )
 
-from git.types import Commit_ish, PathLike, TBD
-
-try:
+if sys.version_info >= (3, 8):
     from typing import Literal
-except ImportError:
+else:
     from typing_extensions import Literal
 
+from git.types import Commit_ish, PathLike, TBD
+
 if TYPE_CHECKING:
     from git.index import IndexFile
     from git.objects.commit import Commit

git/objects/tag.py

@@ -9,16 +9,18 @@
 For lightweight tags, see the :mod:`git.refs.tag` module.
 """
 
+import sys
+
 from . import base
 from .util import get_object_type_by_name, parse_actor_and_date
 from ..util import hex_to_bin
 from ..compat import defenc
 
 from typing import List, TYPE_CHECKING, Union
 
-try:
+if sys.version_info >= (3, 8):
     from typing import Literal
-except ImportError:
+else:
     from typing_extensions import Literal
 
 if TYPE_CHECKING:

git/objects/tree.py

@@ -3,17 +3,16 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
-from git.util import IterableList, join_path
+import sys
+
 import git.diff as git_diff
-from git.util import to_bin_sha
+from git.util import IterableList, join_path, to_bin_sha
 
-from . import util
-from .base import IndexObject, IndexObjUnion
+from .base import IndexObjUnion, IndexObject
 from .blob import Blob
-from .submodule.base import Submodule
-
 from .fun import tree_entries_from_data, tree_to_stream
-
+from .submodule.base import Submodule
+from . import util
 
 # typing -------------------------------------------------
 
@@ -25,22 +24,22 @@
     Iterator,
     List,
     Tuple,
+    TYPE_CHECKING,
     Type,
     Union,
     cast,
-    TYPE_CHECKING,
 )
 
-from git.types import PathLike
-
-try:
+if sys.version_info >= (3, 8):
     from typing import Literal
-except ImportError:
+else:
     from typing_extensions import Literal
 
+from git.types import PathLike
+
 if TYPE_CHECKING:
-    from git.repo import Repo
     from io import BytesIO
+    from git.repo import Repo
 
 TreeCacheTup = Tuple[bytes, int, str]