Let's say a portfolio manager asks you to build a model to forecast the daily conditional variance of a specific stock. Walk me through the mathematical formulation of a GARCH(1,1) model, and how you would go about estimating its parameters using Python.

某投资组合经理要求你建立一个预测特定股票日条件方差的模型。请详细阐述GARCH(1,1)模型的数学表达式，并说明如何使用Python实现参数估计。