Switch from pipfile to plette lib

The main user of `pipfile` is `pipenv`, and _not_ any of the other
python package managers.

However, `pipfile` library has been pretty much unmaintained, so
`pipenv` switched to using `plette` for parsing/validation of
`Pipfile`'s:
* pypa/pipenv#5310
* pypa/pipenv#5339

So let's switch our usage as well. Today we only use `pipfile` for
generating hashes, so this is effectively a silent no-op. However, down
the road we could leverage `plette` for `Pipfile` parsing/validation...
for example see how it's flagging things here:
#6104 (comment)

python/helpers/lib/hasher.py

@@ -1,6 +1,6 @@
 import hashin
 import json
-import pipfile
+import plette
 from poetry.factory import Factory
 
 
@@ -15,9 +15,10 @@ def get_dependency_hash(dependency_name, dependency_version, algorithm):
 
 
 def get_pipfile_hash(directory):
-    p = pipfile.load(directory + '/Pipfile')
+    with open(directory + '/Pipfile') as f:
+        pipfile = plette.Pipfile.load(f)
 
-    return json.dumps({"result": p.hash})
+    return json.dumps({"result": pipfile.get_hash().value})
 
 
 def get_pyproject_hash(directory):

python/helpers/requirements.txt

@@ -2,7 +2,7 @@ pip>=21.3.1,<23.2.0  # Range maintains py36 support TODO: Review python 3.6 supp
 pip-tools>=6.4.0,<=6.14.0  # Range maintains py36 support TODO: Review python 3.6 support in April 2023 (eol ubuntu 18.04)
 hashin==0.17.0
 pipenv==2022.4.8
-pipfile==0.0.2
+plette==0.4.4
 poetry>=1.1.15,<1.6.0
 # For now we chose to pin `wheel` even though we don't import it directly.
 # Background context: https://github.com/dependabot/dependabot-core/pull/5597