fix unsorted series for get_stat_series

[manojbalaji1]

Issue raised: series should be sorted

Changes

changes in file: datacommons_pandas/df_builder.py
changes in line: L49, L53

Change Description:
changed the return statement of function build_time_series by adding a suffixing .sort_index()
also changed the docstring last line from " representing a time series satisfying all optional args." to "representing a sorted time series satisfying all optional args."

Test Result

platform darwin -- Python 3.7.10, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
rootdir: /Users/manojbalaji/personal/api-python
collected 45 items

datacommons/test/core_test.py ........... [ 24%]
datacommons/test/places_test.py ......... [ 44%]
datacommons/test/populations_test.py ........ [ 62%]
datacommons/test/query_test.py .. [ 66%]
datacommons/test/set_api_key_test.py .... [ 75%]
datacommons/test/stat_vars_test.py ...... [ 88%]
datacommons_pandas/test/df_builder_test.py ..... [100%]

Note: Not tested for Python2 since its deprecated

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[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[manojbalaji1]

@googlebot I fixed it.

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[manojbalaji1]

closed it as google-cla didn't work, recreated again