take into account that no Python dependency may be provided when setting $Python3_ROOT_DIR in configure step of PythonPackage easyblock

[boegel]

(created using eb --new-pr)

fix for bug that was introduced in:

• Set $Python3_ROOT_DIR in PythonPackage easyblock #4043

  File "/home/example/easybuild/easyblocks/generic/pythonpackage.py", line 923, in configure_step
    env.setvar("Python3_ROOT_DIR", get_software_root('Python'))
  File "/home/example/easybuild/tools/environment.py", line 92, in setvar
    os.environ[key] = value
  File "/usr/lib64/python3.9/os.py", line 684, in __setitem__
    value = self.encodevalue(value)
  File "/usr/lib64/python3.9/os.py", line 756, in encode
    raise TypeError("str expected, not %s" % type(value).__name__)
TypeError: str expected, not NoneType

In some cases, there is no Python dependency. One example is vsc-mympirun-5.5.0.eb

[laraPPr]

lgtm

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS vsc-mympirun-5.5.0.eb

• SUCCESS matplotlib-3.7.2-gfbf-2023a.eb

Build succeeded for 2 out of 2 (total: 4 mins 36 secs) (2 easyconfigs in total)
node4009.donphan.os - Linux RHEL 9.6, x86_64, Intel(R) Xeon(R) Gold 6240 CPU @ 2.60GHz (cascadelake), 1 x NVIDIA NVIDIA A2, 580.95.05, Python 3.9.21
See https://gist.github.com/boegel/289cebeba8d8d70edf67ad16c876518f for a full test report.

[ocaisa]

LGTM

[laraPPr]

Going in, thanks @boegel!