[AIRFLOW-2060] Update pendulum version to 1.4.4

This fixes a task clearing issue with deep copy

Closes #3154 from cinhil/AIRFLOW-2060

setup.py

@@ -235,7 +235,7 @@ def do_setup():
             'lxml>=3.6.0, <4.0',
             'markdown>=2.5.2, <3.0',
             'pandas>=0.17.1, <1.0.0',
-            'pendulum==1.4.0',
+            'pendulum==1.4.4',
             'psutil>=4.2.0, <5.0.0',
             'pygments>=2.0.1, <3.0',
             'python-daemon>=2.1.1, <2.2',