Increase minimum Python version required, to “>= 3.7”.

`setuptools.command.build` was introduced since 62.4.0 (see
pypa/wheel#487 (comment)), which
require Python >= 3.7.

Fixes https://pagure.io/python-daemon/issue/74

setup.py

@@ -74,7 +74,7 @@
             "packaging",
             "lockfile >=0.10",
             ],
-        python_requires=">=3",
+        python_requires=">=3.7",
         extras_require={
             'test': test_requirements,
             'build': build_requirements,