Unpin docstring-parser in requirements.txt

[kiukchung]

Resolves: #771

Test plan:

Ran unittests after upgrading docstring-parser to the latest (0.15)

$ pip uninstall docstring-parser
$ pip install -U -r requirements.txt
$ pip show docstring-parser
Name: docstring-parser
Version: 0.15
Summary: Parse Python docstrings in reST, Google and Numpydoc format
Home-page: https://github.com/rr-/docstring_parser
Author: Marcin Kurczewski
Author-email: dash@wind.garden
License: MIT
Location: /usr/local/google/home/kiuk/.pyenv/versions/3.10.12/envs/torchx_310/lib/python3.10/site-packages
Requires:
Required-by: kfp

$ pytest
<... TEST LOG OUTPUT OMITTED FOR BREVITY ...>
================================================================================================ 618 passed, 90 warnings in 90.51s (0:01:30) ================================================================================================

[codecov]

Codecov Report

Merging #772 (be8a5de) into main (d713116) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #772   +/-   ##
=======================================
  Coverage   92.88%   92.88%           
=======================================
  Files          96       96           
  Lines        6098     6098           
=======================================
  Hits         5664     5664           
  Misses        434      434           

Flag	Coverage Δ
unittests	92.88% <ø> (ø)

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[d4l3k]

LGTM