Removing invalid BSD-3 Clause license classifier.

According to the pypi list of standardized classifiers, the classifier
`License :: OSI Approved :: BSD-3 Clause` is invalid.

This is also made clear by looking at the two latest publish package
steps, which both failed with the following error:

```
HTTPError: 400 Bad Request from https://upload.pypi.org/legacy/
Invalid value for classifiers. Error: Classifier 'License :: OSI
Approved :: BSD-3 Clause' is not a valid classifier.
```

Removing this classifier should make the package valid for publishing
again as all remaining classifiers are valid.

See: https://pypi.org/classifiers/

docs/changes.rst

@@ -1,6 +1,10 @@
 Changelog
 *********
 
+1.8.4
+=====
+* Removing invalid BSD-3 Clause license classifier.
+
 1.8.3
 =====
 * Changed `__iter__` return type to `Iterator`.

pyproject.toml

@@ -13,7 +13,6 @@ readme = "README.rst"
 authors = [{name = "Data Engineering Collective", email = "[email protected]"}]
 classifiers = [
     "License :: OSI Approved :: BSD License",
-    "License :: OSI Approved :: BSD-3 Clause",
     "Programming Language :: Python :: 3",
     "Programming Language :: Python :: 3.8",
     "Programming Language :: Python :: 3.9",