requirements.txt: Require pynwb 3.1.2

This makes the maintenance burden smaller as both IPNWB, das
data-upstream, and ipfx then use the same nwb-schema version (2.9.0).

Author: t-b

requirements.txt

@@ -9,7 +9,7 @@ pandas
 pg8000
 pillow
 pyabf
-pynwb
+pynwb==3.1.2
 pyYAML
 ruamel.yaml<0.18.0
 scipy