Fix numpy version for py39 (#1963)

Author: rly

requirements.txt

@@ -1,6 +1,7 @@
 # pinned dependencies to reproduce an entire development environment to use PyNWB
 h5py==3.11.0
 hdmf==3.14.3
-numpy==2.1.1
+numpy==2.1.1; python_version > "3.9"  # numpy 2.1+ is not compatible with py3.9
+numpy==2.0.2; python_version == "3.9"
 pandas==2.2.2
 python-dateutil==2.9.0.post0