Replace the use of `assert` in non-test code (#80)

* Replace `assert`s in the `conversable_agent` module with `if-log-raise`.

* Use a `logger` object in the `code_utils` module.

* Replace use of `assert` with `if-log-raise` in the `code_utils` module.

* Replace use of `assert` in the `math_utils` module with `if-not-raise`.

* Replace `assert` with `if` in the `oai.completion` module.

* Replace `assert` in the `retrieve_utils` module with an if statement.

* Add missing `not`.

* Blacken `completion.py`.

* Test `generate_reply` and `a_generate_reply` raise an assertion error
when there are neither `messages` nor a `sender`.

* Test `execute_code` raises an `AssertionError` when neither code nor
filename is provided.

* Test `split_text_to_chunks` raises when passed an invalid chunk mode.

* * Add `tiktoken` and `chromadb` to test dependencies as they're used in
the `test_retrieve_utils` module.

* Sort the test requirements alphabetically.