Upgrade PyYAML 5.1.2 to 5.3.1

https://github.com/yaml/pyyaml/blob/d0d660d035905d9c49fc0f8dafb579d2cc68c0c8/CHANGES#L7

5.3.1 (2020-03-18)

* yaml/pyyaml#386 -- Prevents arbitrary code execution during python/object/new constructor

5.3 (2020-01-06)

* yaml/pyyaml#290 -- Use `is` instead of equality for comparing with `None`
* yaml/pyyaml#270 -- fix typos and stylistic nit
* yaml/pyyaml#309 -- Fix up small typo
* yaml/pyyaml#161 -- Fix handling of __slots__
* yaml/pyyaml#358 -- Allow calling add_multi_constructor with None
* yaml/pyyaml#285 -- Add use of safe_load() function in README
* yaml/pyyaml#351 -- Fix reader for Unicode code points over 0xFFFF
* yaml/pyyaml#360 -- Enable certain unicode tests when maxunicode not > 0xffff
* yaml/pyyaml#359 -- Use full_load in yaml-highlight example
* yaml/pyyaml#244 -- Document that PyYAML is implemented with Cython
* yaml/pyyaml#329 -- Fix for Python 3.10
* yaml/pyyaml#310 -- increase size of index, line, and column fields
* yaml/pyyaml#260 -- remove some unused imports
* yaml/pyyaml#163 -- Create timezone-aware datetimes when parsed as such
* yaml/pyyaml#363 -- Add tests for timezone

5.2 (2019-12-02)
------------------

* Repair incompatibilities introduced with 5.1. The default Loader was changed,
  but several methods like add_constructor still used the old default
  yaml/pyyaml#279 -- A more flexible fix for custom tag constructors
  yaml/pyyaml#287 -- Change default loader for yaml.add_constructor
  yaml/pyyaml#305 -- Change default loader for add_implicit_resolver, add_path_resolver
* Make FullLoader safer by removing python/object/apply from the default FullLoader
  yaml/pyyaml#347 -- Move constructor for object/apply to UnsafeConstructor
* Fix bug introduced in 5.1 where quoting went wrong on systems with sys.maxunicode <= 0xffff
  yaml/pyyaml#276 -- Fix logic for quoting special characters
* Other PRs:
  yaml/pyyaml#280 -- Update CHANGES for 5.1

3rdparty/python/requirements.txt

@@ -30,7 +30,7 @@ pyopenssl==17.3.0
 pystache==0.5.3
 python-Levenshtein==0.12.0
 pywatchman==1.4.1
-PyYAML==5.1.2
+PyYAML==5.3.1
 py_zipkin==0.18.4
 requests[security]>=2.20.1
 responses==0.10.4