[AIRFLOW-XXX] Pin pinodb dependency to fix the existing issue

[feng-tao]

pinodb 0.2.2 has a breaking change which breaks the unit test. Pin the dependency for now.

47) ERROR: Failure: SyntaxError (invalid syntax (db.py, line 41))
----------------------------------------------------------------------
   Traceback (most recent call last):
    .tox/py27-backend_mysql-env_docker/lib/python2.7/site-packages/nose/loader.py line 418 in loadTestsFromName
      addr.filename, addr.module)
    .tox/py27-backend_mysql-env_docker/lib/python2.7/site-packages/nose/importer.py line 47 in importFromPath
      return self.importFromDir(dir_path, fqname)
    .tox/py27-backend_mysql-env_docker/lib/python2.7/site-packages/nose/importer.py line 94 in importFromDir
      mod = load_module(part_fqname, fh, filename, desc)
    tests/contrib/hooks/test_pinot_hook.py line 24 in <module>
      from airflow.contrib.hooks.pinot_hook import PinotDbApiHook
    airflow/contrib/hooks/pinot_hook.py line 22 in <module>
      from pinotdb import connect
    .tox/py27-backend_mysql-env_docker/lib/python2.7/site-packages/pinotdb/__init__.py line 1 in <module>
      from pinotdb.db import connect
   SyntaxError: invalid syntax (db.py, line 41)```

[feng-tao]

PTAL @ashb @ffinfo

[feng-tao]

And @kaxil @Fokko

[kaxil]

LGTM as long as CI passes

[codecov-io]

Codecov Report

Merging #4704 into master will increase coverage by 63.37%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #4704       +/-   ##
==========================================
+ Coverage   11.03%   74.4%   +63.37%     
==========================================
  Files         430     430               
  Lines       27971   27971               
==========================================
+ Hits         3087   20813    +17726     
+ Misses      24884    7158    -17726

Impacted Files	Coverage Δ
airflow/plugins_manager.py	86.66% <0%> (+0.95%)	⬆️
airflow/executors/dask_executor.py	2% <0%> (+2%)	⬆️
airflow/exceptions.py	100% <0%> (+2.85%)	⬆️
airflow/utils/operator_resources.py	86.95% <0%> (+4.34%)	⬆️
...etes_request_factory/kubernetes_request_factory.py	72.82% <0%> (+5.43%)	⬆️
airflow/executors/__init__.py	63.46% <0%> (+5.76%)	⬆️
airflow/utils/module_loading.py	100% <0%> (+10.52%)	⬆️
airflow/settings.py	80.88% <0%> (+13.23%)	⬆️
airflow/utils/decorators.py	91.66% <0%> (+14.58%)	⬆️
airflow/hooks/oracle_hook.py	15.47% <0%> (+15.47%)	⬆️
... and 329 more

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