Add `PendingDeprecationWarning` to the existing library

[JP-Ellis]

As part of the broader migration to FFI, we will be deprecating the existing library. We want to make sure that end-users are aware of this upcoming change and can appropriately prepare for it.

The PendingDeprecationWarning is a warning that is not raised at runtime, and only is enabled by default in Python during development mode.

[github-actions]

👋 Hi! The 'smartbear-supported' label has just been added to this issue, which will create an internal tracking ticket in PactFlow's Jira (PACT-1354). We will use this to prioritise and assign a team member to this task. All activity will be public on this ticket. For now, sit tight and we'll update this ticket once we have more information on the next steps.

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