Adding requirements.txt into MANIFEST.in #398

Job	Run time
Docs	1m 58s
CI (macos-15-intel, 3.12)	24s
CI (ubuntu-latest, 3.10)	21s
CI (macos-15-intel, 3.10)	30s
CI (macos-latest, 3.9)	34s
CI (macos-latest, 3.11)	29s
CI (windows-latest, 3.9)	52s
CI (macos-latest, 3.10)	46s
Integration (macos-15-intel, 3.10)	56s
CI (windows-latest, 3.12)	47s
CI (ubuntu-latest, 3.12)	17s
Integration (macos-latest, 3.9)	54s
CI (ubuntu-latest, 3.11)	20s
Integration (macos-15-intel, 3.9)	49s
CI (ubuntu-latest, 3.9)	26s
CI (macos-latest, 3.12)	17s
CI (macos-15-intel, 3.9)	35s
Integration (macos-15-intel, pypy-3.8)	1m 10s
CI (windows-latest, 3.10)	51s
CI (windows-latest, 3.11)	54s
Integration (macos-latest, 3.11)	39s
Integration (macos-15-intel, pypy-3.10)	1m 3s
Integration (macos-latest, 3.10)	2m 0s
Integration (macos-latest, pypy-3.8)	2m 34s
Integration (windows-latest, 3.9)	1m 6s
Integration (windows-latest, 3.10)	1m 23s
CI (macos-15-intel, 3.11)	28s
Integration (macos-latest, pypy-3.10)	1m 2s
Integration (windows-latest, pypy-3.8)	12s
Integration (windows-latest, 3.11)	1m 3s
Integration (macos-15-intel, 3.11)	46s
Integration (ubuntu-latest, 3.9)	30s
Integration (ubuntu-latest, pypy-3.10)	1m 11s
Integration (ubuntu-latest, pypy-3.8)	48s
Integration (ubuntu-latest, 3.10)	42s
Integration (ubuntu-latest, 3.11)	38s
Integration (windows-latest, pypy-3.10)	2m 29s
	32m 44s

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