Change license headers from comments to strings

[oleksandr-pavlyk]

So far we have adopted stating license governing Python code as Python comments.

Project loopy states the license as a string in __license__ module attribute, e.g. https://github.com/inducer/loopy/blob/main/loopy/compiled.py#L3

Following loopy, we could also define __copyright__ attribute.

The advantage, particularly for compile extensions, is that license can be dynamically retrieved in a Python session by any user, and presence of the license string can be tested in our test-suite.