README: Update badges

GitHub changed its rendering, so badged have been rendered with line-
breaks, causing unpleasant appearance.

README.rst

@@ -1,36 +1,43 @@
-.. image:: https://github.com/mqtt-tools/pytest-mqtt/actions/workflows/tests.yml/badge.svg
+###########
+pytest-mqtt
+###########
+
+|
+
+.. start-badges
+
+|ci-tests| |ci-coverage| |license| |pypi-downloads|
+|python-versions| |status| |pypi-version|
+
+.. |ci-tests| image:: https://github.com/mqtt-tools/pytest-mqtt/actions/workflows/tests.yml/badge.svg
     :target: https://github.com/mqtt-tools/pytest-mqtt/actions/workflows/tests.yml
     :alt: CI outcome
 
-.. image:: https://codecov.io/gh/mqtt-tools/pytest-mqtt/branch/main/graph/badge.svg
+.. |ci-coverage| image:: https://codecov.io/gh/mqtt-tools/pytest-mqtt/branch/main/graph/badge.svg
     :target: https://codecov.io/gh/mqtt-tools/pytest-mqtt
     :alt: Test suite code coverage
 
-.. image:: https://pepy.tech/badge/pytest-mqtt/month
+.. |pypi-downloads| image:: https://pepy.tech/badge/pytest-mqtt/month
     :target: https://pepy.tech/project/pytest-mqtt
     :alt: PyPI downloads per month
 
-.. image:: https://img.shields.io/pypi/v/pytest-mqtt.svg
+.. |pypi-version| image:: https://img.shields.io/pypi/v/pytest-mqtt.svg
     :target: https://pypi.org/project/pytest-mqtt/
     :alt: Package version on PyPI
 
-.. image:: https://img.shields.io/pypi/status/pytest-mqtt.svg
+.. |status| image:: https://img.shields.io/pypi/status/pytest-mqtt.svg
     :target: https://pypi.org/project/pytest-mqtt/
     :alt: Project status (alpha, beta, stable)
 
-.. image:: https://img.shields.io/pypi/pyversions/pytest-mqtt.svg
+.. |python-versions| image:: https://img.shields.io/pypi/pyversions/pytest-mqtt.svg
     :target: https://pypi.org/project/pytest-mqtt/
     :alt: Supported Python versions
 
-.. image:: https://img.shields.io/pypi/l/pytest-mqtt.svg
+.. |license| image:: https://img.shields.io/pypi/l/pytest-mqtt.svg
     :target: https://github.com/mqtt-tools/pytest-mqtt/blob/main/LICENSE
     :alt: Project license
 
-|
-
-###########
-pytest-mqtt
-###########
+.. end-badges
 
 
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