build: Remove declared support for Python<2.7

Master and the 0.3.x branch have never supported these versions, but we didn't
update the metadata.

setup.py

@@ -60,7 +60,7 @@ setup(
     license = 'New BSD',
     url = 'https://github.com/mitogen-hq/mitogen/',
     packages = find_packages(exclude=['tests', 'examples']),
-    python_requires='>=2.4, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*, <4',
+    python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*',
     zip_safe = False,
     classifiers = [
         'Environment :: Console',
@@ -70,10 +70,6 @@ setup(
         'Operating System :: MacOS :: MacOS X',
         'Operating System :: POSIX',
         'Programming Language :: Python',
-        'Programming Language :: Python :: 2',
-        'Programming Language :: Python :: 2.4',
-        'Programming Language :: Python :: 2.5',
-        'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.6',