Add monthly downloads badge to README #470

Job	Run time
Python 3.6 on macos-13	2m 42s
Linux docs	1m 15s
Python 3.12 on macos-13	3m 18s
Python 3.6 on Windows x86	2m 52s
Linux meta	1m 9s
Python 3.12 (minimal build) on macos-13	4m 14s
Python 3.7 on Windows x86	2m 45s
Linux mypy	57s
Python 3.12 on macos-latest	2m 2s
Python 3.8 on Windows x86	2m 49s
Linux pypy3	1m 30s
Python 3.12 (minimal build) on macos-latest	1m 27s
Python 3.9 on Windows x86	2m 45s
Linux pypy3	1m 31s
Python 3.10 on Windows x86	2m 49s
Linux py36	1m 31s
Python 3.11 on Windows x86	2m 44s
Linux py37	1m 30s
Python 3.12 on Windows x86	2m 7s
Linux py38	1m 28s
Python 3.6 on Windows x64	2m 13s
Linux py39	1m 31s
Python 3.7 on Windows x64	2m 16s
Linux py310	1m 19s
Python 3.8 on Windows x64	2m 15s
Linux py311	1m 30s
Python 3.9 on Windows x64	2m 10s
Linux py312	1m 19s
Python 3.10 on Windows x64	2m 28s
Linux py312 system	1m 31s
Python 3.11 on Windows x64	2m 10s
Python 3.12 on Windows x64	1m 53s
Linux py312 minimal	1m 22s
	1h 7m 22s

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