## What changes were proposed in this pull request?
After upgrading cloudpickle to 0.6.1 at https://github.com/apache/spark/pull/20691, one regression was found. Cloudpickle had a critical https://github.com/cloudpipe/cloudpickle/pull/240 for that.
Basically, it currently looks existing globals would override globals shipped in a function's, meaning:
**Before:**
```python
>>> def hey():
... return "Hi"
...
>>> spark.range(1).rdd.map(lambda _: hey()).collect()
['Hi']
>>> def hey():
... return "Yeah"
...
>>> spark.range(1).rdd.map(lambda _: hey()).collect()
['Hi']
```
**After:**
```python
>>> def hey():
... return "Hi"
...
>>> spark.range(1).rdd.map(lambda _: hey()).collect()
['Hi']
>>>
>>> def hey():
... return "Yeah"
...
>>> spark.range(1).rdd.map(lambda _: hey()).collect()
['Yeah']
```
Therefore, this PR upgrades cloudpickle to 0.8.0.
Note that cloudpickle's release cycle is quite short.
Between 0.6.1 and 0.7.0, it contains minor bug fixes. I don't see notable changes to double check and/or avoid.
There is virtually only this fix between 0.7.0 and 0.8.1 - other fixes are about testing.
## How was this patch tested?
Manually tested, tests were added. Verified unit tests were added in cloudpickle.
Closes#23904 from HyukjinKwon/SPARK-27000.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
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