[setup.py] Use PEP625-compliant lowercase 'brotli' package name

PyPI now requires that wheel filenames and metadata use normalized
package names following PEP 625. Modern build tooling automatically
normalize this, however this does not happen for old python versions
(e.g. 2.7, 3.6, 3.7) which makes their upload fail.
I had to apply the same patch to the v1.2.0 release be able to
upload all the python wheels to PyPI, see:

#1327 (comment)

google/brotli-wheels@ca4fed1

It makes more sense to have this in here and get rid of the patch,
regardless of when we drop support for those EOL pythons.

Author: anthrotype

setup.py

@@ -140,7 +140,7 @@ def build_extension(self, ext):
         )
 
 
-NAME = "Brotli"
+NAME = "brotli"
 
 VERSION = get_version()