Add __reduce__ to Body class

[Yash-10]

This change does seem to solve the problem mentioned in #1395. __reduce__ returns a string, which I think is commonly done for singleton classes. As a result, I was able to get the plot:

Just wondering if this is really a solution to the issue...

[codecov]

Codecov Report

Merging #1443 (9ca7563) into main (93f9fab) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1443   +/-   ##
=======================================
  Coverage   91.83%   91.84%           
=======================================
  Files          95       95           
  Lines        4448     4450    +2     
  Branches      430      430           
=======================================
+ Hits         4085     4087    +2     
  Misses        273      273           
  Partials       90       90           

Impacted Files	Coverage Δ
src/poliastro/bodies.py	85.71% <100.00%> (+0.52%)	⬆️

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[astrojuanlu]

Looks like __reduce__ accepts either a string, and that it's the right thing to do in this case:

If a string is returned, the string should be interpreted as the name of a global variable. It should be the object’s local name relative to its module; the pickle module searches the module namespace to determine the object’s module. This behaviour is typically useful for singletons.

https://docs.python.org/3/library/pickle.html#object.__reduce__

So, this solution is good! But it needs a test 😄

[Yash-10]

I used the tmp_path fixture (https://docs.pytest.org/en/latest/how-to/tmp_path.html#the-tmp-path-fixture) to save and load objects into a file for testing. Had not used it before though. Hopefully it's not an overkill 😅

[astrojuanlu]

Looks perfect!