Steps to reproduce

Install PDM with pipx using Python 3.9:

pipx install pdm --python python3.9

In a project, run pdm use -f 3.6, then pdm install -d:

Actual behavior

% pdm install -d 
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/pep508.py", line 1
    from __future__ import annotations
    ^
SyntaxError: future feature annotations is not defined
[CalledProcessError]: Command '['/home/pawamoy/.basher-packages/pyenv/pyenv/versions/3.6.9/bin/python', '-I', '/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/pep508.py']' returned non-zero exit status 1.
Add '-v' to see the detailed traceback

% pdm install -dv
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/pep508.py", line 1
    from __future__ import annotations
    ^
SyntaxError: future feature annotations is not defined
Traceback (most recent call last):
  File "/home/pawamoy/.local/bin/pdm", line 8, in <module>
    sys.exit(main())
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/core.py", line 179, in main
    return Core().main(args)
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/core.py", line 142, in main
    raise err.with_traceback(traceback)
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/core.py", line 137, in main
    f(options.project, options)
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/cli/commands/install.py", line 40, in handle
    actions.do_sync(
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/cli/actions.py", line 127, in do_sync
    handler.synchronize()
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/installers/synchronizers.py", line 263, in synchronize
    to_add, to_update, to_remove = self.compare_with_working_set()
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/installers/synchronizers.py", line 118, in compare_with_working_set
    environment = self.environment.marker_environment
  File "/usr/lib/python3.9/functools.py", line 969, in __get__
    val = self.func(instance)
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/models/environment.py", line 330, in marker_environment
    return get_pep508_environment(self.python_executable)
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/models/in_process.py", line 63, in get_pep508_environment
    return json.loads(subprocess.check_output(args))
  File "/usr/lib/python3.9/subprocess.py", line 424, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.9/subprocess.py", line 528, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/home/pawamoy/.basher-packages/pyenv/pyenv/versions/3.6.9/bin/python', '-I', '/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/pep508.py']' returned non-zero exit status 1.

Expected behavior

I hope PDM will continue to support Python versions less than 3.7!

Environment Information

% pdm info && pdm info --env
PDM version:        1.5.0b0                                                                     
Python Interpreter: /home/pawamoy/.basher-packages/pyenv/pyenv/versions/3.6.9/bin/python (3.6.9)
Project Root:       /media/data/dev/pawamoy/copier-templates-extensions                         
Project Packages:   /media/data/dev/pawamoy/copier-templates-extensions/__pypackages__/3.6      
  File "/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/pep508.py", line 1
    from __future__ import annotations
    ^
SyntaxError: future feature annotations is not defined
[CalledProcessError]: Command '['/home/pawamoy/.basher-packages/pyenv/pyenv/versions/3.6.9/bin/python', '-I', '/home/pawamoy/.local/pipx/venvs/pdm/lib/python3.9/site-packages/pdm/pep508.py']' returned non-zero exit status 1.
Add '-v' to see the detailed traceback

😱

% pdm use -f 3.7            
Using Python interpreter: /home/pawamoy/.basher-packages/pyenv/pyenv/versions/3.7.5/bin/python (3.7.5)
Updating executable scripts...

% pdm info && pdm info --env
PDM version:        1.5.0b0                                                                     
Python Interpreter: /home/pawamoy/.basher-packages/pyenv/pyenv/versions/3.7.5/bin/python (3.7.5)
Project Root:       /media/data/dev/pawamoy/copier-templates-extensions                         
Project Packages:   /media/data/dev/pawamoy/copier-templates-extensions/__pypackages__/3.7      
{
  "implementation_name": "cpython",
  "implementation_version": "3.7.5",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "5.11.11-arch1-1",
  "platform_system": "Linux",
  "platform_version": "#1 SMP PREEMPT Tue, 30 Mar 2021 14:10:17 +0000",
  "python_full_version": "3.7.5",
  "platform_python_implementation": "CPython",
  "python_version": "3.7",
  "sys_platform": "linux"
}