Add Python model support for MySQL (pandas & PySpark)

[tripleaceme]

Summary

• Adds Python model support for MySQL 8.0+ with two execution backends: pandas (default, via SQLAlchemy) and PySpark (via JDBC)
• Python models run locally as subprocesses and write results back to MySQL
• Supports both table and incremental materializations with Python language

Implementation Details

New Files

• python_submissions.py - PandasPythonJobHelper and PySparkPythonJobHelper
• py_write_table macro for both backends
• Table materialization with supported_languages=['sql', 'python']
• Unit tests for submission helpers

Modified Files

• MySQLAdapter - added Python model properties and methods
• create_table_as macro - accepts language parameter
• Incremental materialization - Python language support
• setup.py - optional extras (python-models, pyspark)
• README.md - Python models documentation

Architecture

• Pandas: model(dbt, engine) returns pd.DataFrame, writes via SQLAlchemy
• PySpark: model(dbt, spark) returns DataFrame, writes via JDBC
• Configurable via submission_method, timeout, and spark_master config

Test plan

• Verify existing SQL models still work (no regressions)
• Test pandas Python model with table and incremental
• Test PySpark Python model with table and incremental
• Test special characters in MySQL password
• Test timeout handling

Closes #180