CI: pin psycopg2 in Linux_py37_cov on 1.2.x (#42073)

Author: simonjayhawkins

ci/deps/actions-37-cov.yaml

@@ -30,7 +30,7 @@ dependencies:
   - openpyxl
   - pandas-gbq
   - google-cloud-bigquery>=1.27.2 # GH 36436
-  - psycopg2
+  - psycopg2=2.8.6
   - pyarrow>=0.15.0
   - pymysql<0.10.0 # temporary pin, GH 36465
   - pytables