feat(python)!: Constrain access to globals from DataFrame.sql in favor of top-level pl.sql

[alexander-beedie]

As discussed; updates df.sql(query) such that it can only access "self", deferring to pl.sql(query) when access to frames in the current stack is required.

This better-distinguishes the purpose of each function and allows for frame-level df.sql to be marginally optimised (as there is no longer any need to introspect globals).

Example

Before:

>>> df1 = pl.DataFrame({"id1": [1, 2]})
>>> df2 = pl.DataFrame({"id2": [3, 4]})
>>> df1.sql("SELECT * FROM df1 CROSS JOIN df2")
shape: (4, 2)
┌─────┬─────┐
│ id1 ┆ id2 │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 3   │
│ 1   ┆ 4   │
│ 2   ┆ 3   │
│ 2   ┆ 4   │
└─────┴─────┘

After:

>>> df1.sql("SELECT * FROM df1 CROSS JOIN df2")
Traceback (most recent call last):
...
polars.exceptions.SQLInterfaceError: relation 'df1' was not found

Use instead:

>>> pl.sql("SELECT * FROM df1 CROSS JOIN df2", eager=True)
shape: (4, 2)
┌─────┬─────┐
│ id1 ┆ id2 │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 3   │
│ 1   ┆ 4   │
│ 2   ┆ 3   │
│ 2   ┆ 4   │
└─────┴─────┘