Cannot resolve simple dependency

[yajo]

• 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

➤ cat pyproject.toml 
[project]
name = ""
version = ""
description = ""
authors = [
    {name = "Jairo Llopis", email = "xxx"},
]
dependencies = [
    
    
]
dev-dependencies = []
requires-python = ">=3.6"
dynamic = ["classifiers"]
license = {text = "MIT"}

[project.urls]
homepage = ""

[tool]
[tool.pdm]

[build-system]
requires = ["pdm-pep517"]
build-backend = "pdm.pep517.api"

➤ pdm add git+https://github.com/pawamoy/pytkdocs.git@master#egg=pytkdocs -v
Adding packages to default dependencies: pytkdocs
======== Start resolving requirements ========
        git+https://github.com/pawamoy/pytkdocs.git@master#egg=pytkdocs
        Adding requirement git+https://github.com/pawamoy/pytkdocs.git@master#egg=pytkdocs
Preparing isolated env for PEP 517 build...
Collecting poetry-core
  Using cached poetry_core-1.0.2-py2.py3-none-any.whl (424 kB)
Installing collected packages: poetry-core
Successfully installed poetry-core-1.0.2
        Adding requirement dataclasses<0.9,>=0.7; python_version == "3.6"(from pytkdocs 0.10.1)
🔒 Lock failed
Unable to find a resolution that satisfies the following requirements:
  dataclasses<0.9,>=0.7; python_version == "3.6" (from <Candidate pytkdocs 0.10.1>)
Please make sure the package names are correct. If so, you can either loosen the version constraints of these dependencies, or set a narrower `requires-python` range in the pyproject.toml.
Traceback (most recent call last):
  File "/var/home/yajo/.local/bin/pdm", line 8, in <module>
    sys.exit(main())
  File "/var/home/yajo/.local/pipx/venvs/pdm/lib64/python3.9/site-packages/pdm/core.py", line 73, in __call__
    return self.main(*args, **kwargs)
  File "/var/home/yajo/.local/pipx/venvs/pdm/lib64/python3.9/site-packages/pdm/core.py", line 112, in main
    raise err.with_traceback(traceback)
  File "/var/home/yajo/.local/pipx/venvs/pdm/lib64/python3.9/site-packages/pdm/core.py", line 108, in main
    f(options.project, options)
  File "/var/home/yajo/.local/pipx/venvs/pdm/lib64/python3.9/site-packages/pdm/cli/commands/add.py", line 35, in handle
    actions.do_add(
  File "/var/home/yajo/.local/pipx/venvs/pdm/lib64/python3.9/site-packages/pdm/cli/actions.py", line 169, in do_add
    resolved = do_lock(project, strategy, tracked_names, reqs)
  File "/var/home/yajo/.local/pipx/venvs/pdm/lib64/python3.9/site-packages/pdm/cli/actions.py", line 69, in do_lock
    mapping, dependencies, summaries = resolve(
  File "/var/home/yajo/.local/pipx/venvs/pdm/lib64/python3.9/site-packages/pdm/resolver/core.py", line 138, in resolve
    result = resolver.resolve(requirements, max_rounds)
  File "/var/home/yajo/.local/pipx/venvs/pdm/lib64/python3.9/site-packages/resolvelib/resolvers.py", line 453, in resolve
    state = resolution.resolve(requirements, max_rounds=max_rounds)
  File "/var/home/yajo/.local/pipx/venvs/pdm/lib64/python3.9/site-packages/resolvelib/resolvers.py", line 357, in resolve
    raise ResolutionImpossible(causes)
resolvelib.resolvers.ResolutionImpossible: [RequirementInformation(requirement=<NamedRequirement dataclasses<0.9,>=0.7; python_version == "3.6">, parent=<Candidate pytkdocs 0.10.1>)]

Actual behavior

pdm cannot resolve this simple dependency requirement: https://github.com/mkdocstrings/pytkdocs/blob/4052eabdd45a7f4fe8c3fc8591bb23e1763a5a0f/pyproject.toml#L24

Expected behavior

It is perfectly resolvable.

Environment Information

# Paste the output of `pdm info && pdm info --env` below:
➤ pdm info && pdm info --env
PDM version:        1.3.0                   
Python Interpreter: /usr/bin/python3 (3.9.1)
Project Root:       /tmp/tmp.mdlDrk53Gi     
{
  "implementation_name": "cpython",
  "implementation_version": "3.9.1",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "5.10.12-200.fc33.x86_64",
  "platform_system": "Linux",
  "platform_version": "#1 SMP Mon Feb 1 02:40:52 UTC 2021",
  "python_full_version": "3.9.1",
  "platform_python_implementaiton": "CPython",
  "python_version": "3.9",
  "sys_platform": "linux"
}