feat(v3): implement pact class

This (rather large) commit implements core functionality for the `Pact`
class, and the `Interaction` class to handle specific interactions
within a Pact.

This does _not_ implement every method that Pacts and/or interactions
might have (e.g., it is currently not possible to specify a Pact
version). The intent of this commit is to be a minimal working example
which can be improved upon more incrementally.

Signed-off-by: JP-Ellis <josh@jpellis.me>

pact/v3/__init__.py

@@ -19,3 +19,10 @@
     library is moved to the `pact.v2` scope. The `pact.v2` module will be
     considered deprecated, and will be removed in a future release.
 """
+
+from .pact import Interaction, Pact
+
+__all__ = [
+    "Pact",
+    "Interaction",
+]