Update dependency dulwich to v0.21.7

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
dulwich	==0.21.6 -> ==0.21.7

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Release Notes

dulwich/dulwich (dulwich)

### [`v0.21.7`](https://togithub.com/jelmer/dulwich/releases/tag/dulwich-0.21.7): 0.21.7 [Compare Source](https://togithub.com/dulwich/dulwich/compare/dulwich-0.21.6...dulwich-0.21.7) - Fix NameError when encountering errors during HTTP operation. (Jelmer Vernooij, [#​1208](https://togithub.com/dulwich/dulwich/issues/1208)) - Raise exception when default identity can't be found. (Jelmer Vernooij) - Add a dedicated exception class for unresolved deltas. (Jelmer Vernooij, [#​1221](https://togithub.com/dulwich/dulwich/issues/1221)) - Support credentials in proxy URL. (Jelmer Vernooij, [#​1227](https://togithub.com/dulwich/dulwich/issues/1227)) - Add `dulwich.porcelain.for_each_ref`. (Daniele Trifirò)

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