Pin pulp<2.8 for snakemake

See parallel conda-base PR for details nextstrain/conda-base#53
Further discussion: https://bedfordlab.slack.com/archives/C01LCTT7JNN/p1705086544738269

Dockerfile

@@ -262,7 +262,8 @@ RUN pip3 install awscli==1.18.195
 
 # Install Snakemake and related optional dependencies.
 # Pinned to 7.32.3 for stability (2023-09-09)
-RUN pip3 install snakemake==7.32.3
+# Pulp>=2.8.0 breaks snakemake <=8.1.1, see https://github.com/snakemake/snakemake/issues/2607
+RUN pip3 install snakemake==7.32.3 "pulp<2.8"
 # Google Cloud Storage package is required for Snakemake to fetch remote files
 # from Google Storage URIs.
 RUN pip3 install google-cloud-storage==2.7.0