Update python:3-slim Docker digest to b877e50 (#67)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| python | container | digest | `63a4c7f` → `b877e50` |

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Author: renovate[bot]

.github/workflows/ci.yml

@@ -46,7 +46,7 @@ jobs:
     name: Indicate completion to coveralls.io
     needs: test
     runs-on: ubuntu-latest
-    container: python:3-slim@sha256:63a4c7f612a00f92042cbdcc7cdc6a306f38485af0a200b9c89de7d9b1607d15
+    container: python:3-slim@sha256:b877e50bd90de10af8d82c57a022fc2e0dc731c5320d762a27986facfc3355c1
     steps:
     - name: Finished
       run: |