Don't use assert to enforce interface constraints

# sscce.py

import git

git.Repo(".").tags[None]

$ git init
Initialized empty Git repository in /tmp/tmp.Xoz9gZndOi/.git/
$ venv/bin/python sscce.py
Traceback (most recent call last):
  File "sscce.py", line 5, in <module>
    git.Repo(".").tags[None]
  File "/tmp/tmp.Xoz9gZndOi/venv/lib/python3.8/site-packages/git/util.py", line 1087, in __getitem__
    assert isinstance(index, (int, str, slice)), "Index of IterableList should be an int or str"
AssertionError: Index of IterableList should be an int or str

assert isinstance(index, (int, str, slice)), "Index of IterableList should be an int or str"

assert should not be used in product code because it can be ignored with -O:

$ venv/bin/python -O sscce.py
Traceback (most recent call last):
  File "sscce.py", line 5, in <module>
    git.Repo(".").tags[None]
  File "/tmp/tmp.Xoz9gZndOi/venv/lib/python3.8/site-packages/git/util.py", line 1095, in __getitem__
    return getattr(self, index)
TypeError: getattr(): attribute name must be string

(In fact, that behavior is probably better since a TypeError is more semantically meaningful.)

bandit can catch this kind of thing:

$ venv/bin/bandit venv/lib/python3.8/site-packages/git/util.py
...
>> Issue: [B101:assert_used] Use of assert detected. The enclosed code will be removed when compiling to optimised byte code.
   Severity: Low   Confidence: High
   CWE: CWE-703 (https://cwe.mitre.org/data/definitions/703.html)
   Location: venv/lib/python3.8/site-packages/git/util.py:1087:8
   More Info: https://bandit.readthedocs.io/en/1.7.4/plugins/b101_assert_used.html
1086
1087            assert isinstance(index, (int, str, slice)), "Index of IterableList should be an int or str"
1088
1089            if isinstance(index, int):
...