Switch to PEP-639 compliant expression of license metadata

PEP 639, and core metadata version 2.4, introduced new ways of
representing a package's license, and deprecated the old License field
and license classifiers. The changes I'm making to the setuptools config
in this commit cause wheels to be generated with the new format of
license metadata.

Author: diazona

setup.py

@@ -15,7 +15,8 @@ def read(fname):
     author_email="[email protected]",
     maintainer="David Zaslavsky",
     maintainer_email="[email protected]",
-    license="MIT License",
+    license_files=["LICENSE"],
+    license_expression="MIT",
     description="pytest plugin to test server connections locally.",
     long_description=read("README.rst"),
     url="https://github.com/pytest-dev/pytest-localserver",
@@ -37,7 +38,6 @@ def read(fname):
         "Operating System :: OS Independent",
         "Development Status :: 4 - Beta",
         "Intended Audience :: Developers",
-        "License :: OSI Approved :: MIT License",
         "Programming Language :: Python :: 3",
         "Programming Language :: Python :: 3 :: Only",
         "Programming Language :: Python :: 3.7",