.dev and .prod requirements files have conflicting requirements

[evansd]

For example:

ehrql/requirements.prod.txt

Lines 7 to 10 in efbed3f

	certifi==2023.5.7 \
	--hash=sha256:0f0d56dc5a6ad56fd4ba36484d6cc34451e1c6548c61daad8c320169f91eddc7 \
	--hash=sha256:c6c2e98f5c7869efca1f8916fed228dd91539f9f1b444c314c06eef02980c716
	# via requests

ehrql/requirements.dev.txt

Lines 39 to 42 in efbed3f

	certifi==2023.7.22 \
	--hash=sha256:539cc1d13202e33ca466e88b2807e29f4c13049d6d87031a3c110744495cb082 \
	--hash=sha256:92d6037539857d8206b8f6ae472e8b77db8058fec5937a1ef3f54304089edbb9
	# via requests

And there are several others, I think maybe all relating to dependencies of requests.

We don't notice this locally because Just installs one after the other and so the dev requirements overwrite the prod ones.

But this seems like it might cause problems at some stage.