[AIRFLOW-5942] Pin PyMSSQL to <3.0 (#6592)

diff --git a/setup.py b/setup.py
index 7dadd5f..2a72785 100644
--- a/setup.py
+++ b/setup.py
@@ -237,7 +237,7 @@
 kubernetes = ['kubernetes>=3.0.0',
               'cryptography>=2.0.0']
 ldap = ['ldap3>=2.5.1']
-mssql = ['pymssql>=2.1.1']
+mssql = ['pymssql~=2.1.1']
 mysql = ['mysqlclient>=1.3.6,<1.4']
 oracle = ['cx_Oracle>=5.1.2']
 papermill = ['papermill[all]>=1.0.0',