Move from LICENSE.md to LICENSE

Wout wrote:

Our license files are typically called "LICENSE.MD" with `# MIT License` as the first line (which makes it markdown). 
Not sure whether PEP639 supports it and whether the first line should be there at all ...

I'm thinking of removing the # character and renaming the file to LICENSE.

Here is a proposition:

license = "MIT"
license-files = ["LICENSE"]
classifiers = [
    "Intended Audience :: Science/Research",
    "Programming Language :: Python :: 3",
]

We should not forget to change the badge URL.