MNT: Drop traits upper bound

[effigies]

This PR:

• Pins numpy<2. Until dipy releases a version compatible with numpy 2, it's not worth it. Numpy 1.26 is the default in present in major distros for now.
• Subclasses traits.BaseTuple to recover the previously lax behavior that allowed Tuple() to check the length and types of arguments without insisting on the container. This fixes the problem that traits. 6.4 introduced, which particularly broke JSON load/save of input specs.
• Small compatibility fix for dipy and traits.
• Drops Python 3.8 support, which is scheduled to be EOL next week.
• Adapts to change in networkx graphviz outputs, which removed a space. It doesn't seem worth making the logic even more complex to test a cosmetic difference on a wide range of networkx versions.

Closes #3661.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.52381% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.83%. Comparing base (4d1352a) to head (f2e9d32). Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
nipype/interfaces/spm/base.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3685 +/- ## ======================================= Coverage 70.83% 70.83% ======================================= Files 1276 1276 Lines 59314 59320 +6 Branches 9824 9826 +2 ======================================= + Hits 42013 42019 +6 Misses 16125 16125 Partials 1176 1176 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.