build(deps): bump service-identity from 18.1.0 to 21.1.0 (#849)

Bumps [service-identity](https://github.com/pyca/service-identity) from 18.1.0 to 21.1.0.
- [Release notes](https://github.com/pyca/service-identity/releases)
- [Changelog](https://github.com/pyca/service-identity/blob/main/CHANGELOG.rst)
- [Commits](https://github.com/pyca/service-identity/compare/18.1.0...21.1.0)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Pipfile.lock

@@ -1101,11 +1101,11 @@
         },
         "service-identity": {
             "hashes": [
-                "sha256:001c0707759cb3de7e49c078a7c0c9cd12594161d3bf06b9c254fdcb1a60dc36",
-                "sha256:0858a54aabc5b459d1aafa8a518ed2081a285087f349fe3e55197989232e2e2d"
+                "sha256:6e6c6086ca271dc11b033d17c3a8bea9f24ebff920c587da090afc9519419d34",
+                "sha256:f0b0caac3d40627c3c04d7a51b6e06721857a0e10a8775f2d1d7e72901b3a7db"
             ],
             "index": "pypi",
-            "version": "==18.1.0"
+            "version": "==21.1.0"
         },
         "six": {
             "hashes": [