relax openai version constraint

Since dspy does in fact support both 0.x and 1.x with [1], the
constraints are updated here to reflect that, allowing users of the
library to make use of whatever version they would like.

[1] stanfordnlp@40c519a (and follow-on work)

pyproject.toml

@@ -21,7 +21,7 @@ classifiers = [
 dependencies = [
     "backoff~=2.2.1",
     "joblib~=1.3.2",
-    "openai~=0.28.1",
+    "openai>=0.28.1,<2.0.0",
     "pandas~=2.1.1",
     "regex~=2023.10.3",
     "ujson~=5.8.0",