Isolated build with 'enscons' as backend build system fails

[oczkoisse]

I'm trying to use tox with isolated_build set to true and enscons as the backend build system. When isolated_build is not set (default is false I think), tox works fine. However, when isolated_build is set to true, the build fails with the following error:

action: .package, msg: get-build-requires
cwd: C:\Users\banga\Projects\Personal\labeling-tool
cmd: 'C:\Users\banga\Projects\Personal\labeling-tool\.tox\.package\Scripts\python' '.venv\Lib\site-packages\tox\helper\build_requires.py' enscons.api '' ''
Traceback (most recent call last):
  File ".venv\Lib\site-packages\tox\helper\build_requires.py", line 15, in <module>
    for_build_requires = backend.get_requires_for_build_sdist(None)
AttributeError: module 'enscons.api' has no attribute 'get_requires_for_build_sdist'

I might be wrong but I thought get_requires_for_build_sdist and get_requires_for_build_wheel were optional hooks in PEP 517 and should be assumed to return empty list if absent.

The tox configuration in pyproject.toml is:

[tool.tox]
legacy_tox_ini = """
[tox]
envlist = py37, py38, py39
isolated_build = true

[testenv]
deps = pytest
commands = pytest
"""

[oczkoisse]

I've set up an example repository here. To reproduce the error:

1. Clone the repo
2. Set up a virtual environment
3. pip install tox enscons
4. Run tox. You should get an error similar to the following

   ========================================================================== log start ==========================================================================
   Traceback (most recent call last):
     File ".venv\Lib\site-packages\tox\helper\build_requires.py", line 15, in <module>
       for_build_requires = backend.get_requires_for_build_sdist(None)
   AttributeError: module 'enscons.api' has no attribute 'get_requires_for_build_sdist'
   
   =========================================================================== log end ===========================================================================
   ERROR: FAIL could not package project - v = InvocationError("'C:\\Users\\banga\\Projects\\Personal\\test-enscons\\.tox\\.package\\Scripts\\python' '.venv\\Lib\\site-packages\\tox\\helper\\build_requires.py' enscons.api '' ''", 1)
   

5. Setting isolated_build = false in pyproject.toml doesn't cause the above error

[gaborbernat]

This is a bug in our system, namely here https://github.com/tox-dev/tox/blob/master/src/tox/helper/build_requires.py#L15. This has been fixed in tox v4, but not on the v3 branch. PRs weclome.