Check Requires-Python before other dependencies

This makes the resolver fail quicker when there's a interpreter
version conflict, and avoid additional (useless) downloads.

Author: uranusjr

news/9925.feature.rst

@@ -0,0 +1,3 @@
+New resolver: A distribution's ``Requires-Python`` metadata is now checked
+before its Python dependencies. This makes the resolver fail quicker when
+there's a interpreter version conflict.

src/pip/_internal/resolution/resolvelib/candidates.py

@@ -256,10 +256,10 @@ def _get_requires_python_dependency(self):
 
     def iter_dependencies(self, with_requires):
         # type: (bool) -> Iterable[Optional[Requirement]]
+        yield self._get_requires_python_dependency()
         requires = self.dist.requires() if with_requires else ()
         for r in requires:
             yield self._factory.make_requirement_from_spec(str(r), self._ireq)
-        yield self._get_requires_python_dependency()
 
     def get_install_requirement(self):
         # type: () -> Optional[InstallRequirement]