[SPARK-32435][PYTHON] Remove heapq3 port from Python 3

### What changes were proposed in this pull request?

This PR removes the manual port of `heapq3.py` introduced from SPARK-3073. The main reason of this was to support Python 2.6 and 2.7 because Python 2's `heapq.merge()` doesn't not support `key` and `reverse`.

See
- https://docs.python.org/2/library/heapq.html#heapq.merge in Python 2
- https://docs.python.org/3.8/library/heapq.html#heapq.merge in Python 3

Since we dropped the Python 2 at SPARK-32138, we can remove this away.

### Why are the changes needed?

To remove unnecessary codes. Also, we can leverage bug fixes made in Python 3.x at `heapq`.

### Does this PR introduce _any_ user-facing change?

No, dev-only.

### How was this patch tested?

Existing tests should cover. I locally ran and verified:

```bash
./python/run-tests --python-executable=python3 --testname="pyspark.tests.test_shuffle"
./python/run-tests --python-executable=python3 --testname="pyspark.shuffle ExternalSorter"
./python/run-tests --python-executable=python3 --testname="pyspark.tests.test_rdd RDDTests.test_external_group_by_key"
```

Closes #29229 from HyukjinKwon/SPARK-32435.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

Author: HyukjinKwon

LICENSE

@@ -222,7 +222,6 @@ external/spark-ganglia-lgpl/src/main/java/com/codahale/metrics/ganglia/GangliaRe
 Python Software Foundation License
 ----------------------------------
 
-pyspark/heapq3.py
 python/docs/source/_static/copybutton.js
 
 BSD 3-Clause

LICENSE-binary

@@ -557,12 +557,6 @@ jakarta.ws.rs:jakarta.ws.rs-api https://github.com/eclipse-ee4j/jaxrs-api
 org.glassfish.hk2.external:jakarta.inject
 
 
-Python Software Foundation License
-----------------------------------
-
-pyspark/heapq3.py
-
-
 Public Domain
 -------------
 

dev/.rat-excludes

@@ -49,7 +49,6 @@ jsonFormatter.min.js
 .*log
 pyspark-coverage-site/*
 cloudpickle/*
-heapq3.py
 join.py
 SparkExprTyper.scala
 SparkILoop.scala

dev/tox.ini

@@ -16,4 +16,4 @@
 [pycodestyle]
 ignore=E226,E241,E305,E402,E722,E731,E741,W503,W504
 max-line-length=100
-exclude=python/pyspark/cloudpickle/*.py,heapq3.py,shared.py,python/docs/source/conf.py,work/*/*.py,python/.eggs/*,dist/*,.git/*
+exclude=python/pyspark/cloudpickle/*.py,shared.py,python/docs/source/conf.py,work/*/*.py,python/.eggs/*,dist/*,.git/*

licenses-binary/LICENSE-heapq.txt

@@ -29,7 +29,7 @@ profile=no
 
 # Add files or directories to the ignoreList. They should be base names, not
 # paths.
-ignore=pyspark.heapq3
+#ignore=
 
 # Pickle collected data for later comparisons.
 persistent=yes