depr(python): Remove re-export of exceptions at top-level

[stinodego]

Removing the top-level re-exports has some minor benefits:

• Unify the way people handle exceptions. Right now we re-export both the exceptions module and the exceptions themselves. We should pick one.
• Reduce clutter in the top-level namespace. This improves IDE hints and is especially relevant as we expand the exception hierarchy.
• Easier to maintain. There are a lot of exceptions - we were already pretty bad at consistently re-exporting all of them, which led to only some exceptions being available top-level, the others only in the exceptions module (I straightened this out recently). This is a bad user experience.

Drawbacks:

• If you have to handle errors, having them available under e.g. pl.ComputeError is quite convenient (though pl.exceptions.ComputeError isn't that much harder).
• There are probably quite a few people using except pl.ComputeError, and this (loudly) breaks their code they will get a deprecation message.

For reference, pandas and NumPy also have a dedicated module for their exceptions and do not re-export at the top-level (NumPy only does for backward compatibility).

Example

Before:

try:
    ...
except pl.ComputeError:
    ...

Use instead:

from polars.exceptions import ComputeError

try:
    ...
except ComputeError:
    ...

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 80.86%. Comparing base (4d15fd4) to head (c8a1b64).
Report is 4 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/dependencies.py	42.85%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17059      +/-   ##
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- Coverage   80.89%   80.86%   -0.03%     
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  Files        1455     1455              
  Lines      190961   191027      +66     
  Branches     2723     2724       +1     
==========================================
  Hits       154473   154473              
- Misses      35984    36050      +66     
  Partials      504      504              

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