Update flake8-builtins requirement from <1.6,>=1.5 to >=1.5,<2.1 in /requirements

[dependabot[bot]]

Updates the requirements on flake8-builtins to permit the latest version.

Changelog

Sourced from flake8-builtins's changelog.

2.0.0 (2022-10-08)

• Mark support for python 3.9. [lovetoburnswhen]

• Replace travis for GitHub. [cclauss]

• Fix CI badge. [sobolevn]

• Make black and isort mandatory. [cclauss]

• Drop python 2.7 and 3.6. [gforcada]

• Overhaul GitHub actions to test on actual supported python versions. [gforcada]

• New flake8 option --builtins-ignorelist to specify a list of builtins to ignore. [gsingh93]

1.5.3 (2020-05-14)

• Change messages to have shorter text

1.5.2 (2020-03-16)

• Add DictComp, SetComp, and GeneratorExpr support. [sobolevn]

1.5.1 (2020-03-16)

• Adds kwonly and posonly args support. [sobolevn]

1.5.0 (2020-03-14)

• Add python3.8 support. [sobolevn]

• Add AnnAssign support. [soboelvn]

• Add NamedExpr or := support. [sobolevn]

1.4.2 (2019-12-18)

... (truncated)

Commits

• 7445a67 Preparing release 2.0.0
• 996eaa3 fix: no need of universal wheels
• b3461fb Merge pull request #86 from gforcada/builtins-whitelist
• 2dff078 Update CHANGES
• 204f348 chore: document the new option
• 81307ec feat(tests): add tests for the new ignore list
• 760030d chore: rename whitelist to ignorelist
• af2a709 Add --builtins-whitelist option
• 552face chore: fix badge URL
• f0220bc Merge pull request #85 from gforcada/drop-py2-py36
• Additional commits viewable in compare view

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