Fix type errors in titer models #3122

Job	Run time
mypy	50s
pathogen-repo-ci (dengue)	2m 47s
check-docs	40s
pathogen-repo-ci (lassa)	7m 16s
pathogen-repo-ci-v0 (avian-flu)	56s
pathogen-repo-ci (measles)	1m 23s
pathogen-repo-ci-v0 (ebola)	54s
pyright	23s
pathogen-repo-ci (mpox)	1m 13s
test (python=3.9 biopython=1.80)	13m 30s
pathogen-repo-ci-v0 (mumps)	56s
pathogen-repo-ci (oropouche)	1m 22s
test (python=3.9 biopython=1.82)	12m 52s
pathogen-repo-ci-v0 (ncov)	1m 9s
pathogen-repo-ci (rabies)	2m 34s
test (python=3.9 biopython=latest)	12m 55s
pathogen-repo-ci-v0 (rsv)	8m 42s
pathogen-repo-ci (seasonal-cov)	1m 53s
test (python=3.10 biopython=1.80)	12m 55s
pathogen-repo-ci-v0 (seasonal-flu)	1m 11s
test (python=3.10 biopython=1.82)	12m 50s
pathogen-repo-ci-v0 (tb)	1m 17s
test (python=3.10 biopython=latest)	12m 53s
pathogen-repo-ci (wnv)	1m 11s
test (python=3.11 biopython=1.80)	12m 34s
pathogen-repo-ci (yellow-fever)	4m 13s
test (python=3.11 biopython=1.82)	12m 45s
pathogen-repo-ci (zika)	1m 4s
test (python=3.11 biopython=latest)	12m 48s
test (python=3.12 biopython=1.82)	19m 33s
test (python=3.12 biopython=latest)	19m 31s
build-docs / build	2m 19s
codecov	10s
release	0s
rebuild-docker-image	0s
	3h 19m 29s

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