feat(python): Improve Schema and DataType interop with Python types

[alexander-beedie]

A few updates that largely finish making interop between Polars/Python types a little smoother and more consistent for the DataType and Schema classes.

• Adds missing conversion/mapping for Struct and Object dtypes.
• Adds a to_python method to both DataType and Schema classes.
• Parses any Python types given to Schema __init__ (or __setitem__).

In addition to making Schema init consistent with the allowed DataFrame/Series dtype init this looser schema definition can also be helpful during EDA, for example, before assigning more exact/specific dtypes in a final production pipeline.

Examples

from datetime import date
import polars as pl

schema = pl.Schema({
    "foo": pl.Int8(),
    "bar": pl.String(),
    "baz": pl.Object(),
    "ham": pl.Categorical("lexical"),
    "spam": pl.Struct({"time": pl.List(pl.Duration), "dist": pl.Float64}),
})

schema.to_python()
# {"foo": int, "bar": str, "baz": object, "ham": str, "spam": dict}

schema = pl.Schema({"foo": int, "bar": str, "baz": object})
schema["ham"] = date
# Schema([('foo', Int64), ('bar', String), ('baz', Object), ('ham', Date)])

pl.DataType.from_python(int)
# pl.Int64

pl.Object().to_python()
# object

[codecov]

Codecov Report

Attention: Patch coverage is 85.18519% with 4 lines in your changes missing coverage. Please review.

Project coverage is 80.34%. Comparing base (70459e4) to head (f246ae5).
Report is 15 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/datatypes/classes.py	71.42%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18308      +/-   ##
==========================================
+ Coverage   80.23%   80.34%   +0.10%     
==========================================
  Files        1501     1501              
  Lines      199180   199236      +56     
  Branches     2837     2841       +4     
==========================================
+ Hits       159822   160078     +256     
+ Misses      38830    38631     -199     
+ Partials      528      527       -1     

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