Fix pypi token regex

What does this MR do?

This MR fixes a bug caused by the pypi token that was recently added which contained a character limit {0,3000}. Golang regexp package only supports length limits up to 1000.

• Follow up on original MR: !104 (comment 542513660)
• Follow up issue to add test for avoiding invalid regexes in gitleaks.toml: gitlab-org/gitlab#326617 (closed)

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Does this MR meet the acceptance criteria?

• x Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer