:zap: Use more threads by default

[ddelange]

Empirically tested on a realistic scenario using Github CI (1physical/2logical core machine), this turned out to be a sensible default:

• 1 thread: 6:40
• 2 threads 4:30
• 4 threads 3:30
• 8 threads 2:30
• 16 threads 2:30

[codecov-commenter]

Codecov Report

Base: 78.30% // Head: 78.30% // No change to project coverage :thumbsup:

Coverage data is based on head (84d2248) compared to base (29acba4). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #106 +/- ## ======================================= Coverage 78.30% 78.30% ======================================= Files 29 29 Lines 2793 2793 Branches 784 784 ======================================= Hits 2187 2187 Misses 436 436 Partials 170 170 ``` | [Impacted Files](https://codecov.io/gh/ddelange/pipgrip/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ddelange) | Coverage Δ | | |---|---|---| | [src/pipgrip/cli.py](https://codecov.io/gh/ddelange/pipgrip/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ddelange#diff-c3JjL3BpcGdyaXAvY2xpLnB5) | `87.01% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ddelange). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ddelange)

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[github-actions[bot]]

Released 0.10.2