[pyproject] Remove Python 3.6 classifier

Author: Olivier Philippon

pyproject.toml

@@ -13,7 +13,6 @@ classifiers = [
     "Operating System :: Microsoft :: Windows",
     "Operating System :: MacOS",
     "Operating System :: POSIX :: Linux",
-    "Programming Language :: Python :: 3.6",
     "Programming Language :: Python :: 3.7",
     "Programming Language :: Python :: 3.8",
     "Programming Language :: Python :: 3.9",