Remove default PYPI URL from PythonBundle ECs

[Flamefire]

This is the default so no need to set it in exts_default_options

[Flamefire]

Test report by @Flamefire
SUCCESS
Build succeeded for 5 out of 5 (5 easyconfigs in total)
n1552 - Linux RHEL 8.9 (Ootpa), x86_64, Intel(R) Xeon(R) Platinum 8470 (icelake), Python 3.8.17
See https://gist.github.com/Flamefire/1c9777eafcf442684e2ac4170869b163 for a full test report.

[Thyre]

We should remove the changes for toolchain 2020b from here, as this one is archived by now.

[Flamefire]

Rebased

[akesandgren]

Test report by @akesandgren
FAILED
Build succeeded for 2 out of 5 (4 easyconfigs in total)
b-cn1611.hpc2n.umu.se - Linux Ubuntu 22.04, x86_64, AMD EPYC 7313 16-Core Processor, 1 x NVIDIA NVIDIA A100 80GB PCIe, 555.58.02, Python 3.10.12
See https://gist.github.com/akesandgren/6194575ec837664e54e43e11343754bb for a full test report.

[Flamefire]

@akesandgren couple failures in the tests. As this removes URLs they are unrelated. Shall we just merge this?

[akesandgren]

Test report by @akesandgren
FAILED
Build succeeded for 3 out of 4 (4 easyconfigs in total)
b-cn1611.hpc2n.umu.se - Linux Ubuntu 22.04, x86_64, AMD EPYC 7313 16-Core Processor, 1 x NVIDIA NVIDIA A100 80GB PCIe, 580.95.05, Python 3.10.12
See https://gist.github.com/akesandgren/fe2b8f825dc65ae610c4b9d7ac0bf5c7 for a full test report.

[Flamefire]

Test report by @Flamefire
SUCCESS
Build succeeded for 4 out of 4 (total: 11 secs) (4 easyconfigs in total)
i7014 - Linux Rocky Linux 9.6, x86_64, AMD EPYC 7702 64-Core Processor (zen2), Python 3.9.21
See https://gist.github.com/Flamefire/e1c2f629d5747c86dfe05401d15d68a9 for a full test report.

[Flamefire]

CuPy fails for me to without this change: #18075 (comment)

The last report was with a forced fetch only so that works and this can be merged.