Add black check into CI and badge in README.md.

Fixes #60.

Author: refeed

.github/workflows/lint.yml

@@ -0,0 +1,14 @@
+name: Linting
+
+on: [push, pull_request]
+
+jobs:
+  black:
+    runs-on: ubuntu-latest
+    name: Black linting
+    steps:
+      - uses: actions/checkout@v3
+      - uses: psf/black@stable
+        with:
+          options: "--check"
+          src: "."

README.md

@@ -1,5 +1,7 @@
 # Zulip HTML archive
 
+[![code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
+
 Generates an HTML archive of a configured set of streams within a
 [Zulip](https://zulip.com) organization (usually all public
 streams).