CI: pin pymysql #36465 (#36847)

* CI: unpin sql to verify the bugs #36465

* CI: pin sqlalchemy

* CI: pin pymsql

* CI: pin sqlalchemy

* CI: pin pymysql

* CI: pin pymysql

* CI: add note

Author: fangchenli

ci/deps/travis-37-cov.yaml

@@ -32,15 +32,15 @@ dependencies:
   - google-cloud-bigquery>=1.27.2 # GH 36436
   - psycopg2
   - pyarrow>=0.15.0
-  - pymysql=0.7.11
+  - pymysql<0.10.0 # temporary pin, GH 36465
   - pytables
   - python-snappy
   - python-dateutil
   - pytz
   - s3fs>=0.4.0
   - scikit-learn
   - scipy
-  - sqlalchemy=1.3.0
+  - sqlalchemy
   - statsmodels
   - xarray
   - xlrd