py2 wheels are preferred when the interpreter is Py3

[frostming]

• I have searched the issue tracker and believe that this is not a duplicate.

Make sure you run commands with -v flag before pasting the output.

Steps to reproduce

pdm init
pdm use 3.7
pdm add futures

Actual behavior

futures-3.1.1-py2-none-any.whl is installed

Expected behavior

futures-3.1.1-py3-none-any.whl is installed

Environment Information

pdm version:

# Paste the output of `pdm info && pdm info --env` below:
Python Interpreter: /usr/local/bin/python3.7m (3.7.6)
Project Root:       /Users/fming/wkspace/github/pdm-test
{
  "implementation_name": "cpython",
  "implementation_version": "3.7.6",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "19.3.0",
  "platform_system": "Darwin",
  "platform_version": "Darwin Kernel Version 19.3.0: Thu Jan  9 20:58:23 PST 2020; root:xnu-6153.81.5~1/RELEASE_X86_64",
  "python_full_version": "3.7.6",
  "platform_python_implementaiton": "CPython",
  "python_version": "3.7",
  "sys_platform": "darwin"
}