Remove pendulum dependency

[massi1008]

Hello,
as discusses over in the pendulum repo that library is "on live support" with the owner being more or less absent.
A big problem is, that there are no wheels for pendulum under Python 3.13. That means that rust/cargo is necessary to install/build pendulum (and in extension fastapi-cache) under that python versions. In my use case that is an additional bothersome complexity; that is why I am exploring ways to remedy this.

fastapi-cache only uses pendulum in one instance, for date/datetime encoding.
Unfortunately I do not fully understand this use case.

1. Why is this encoding here even necessary?
2. Is it possible to remove this encoding, or replace it with something from the main library?

Hopefully pendulum can gain some active maintainers and provide wheels for current and future python versions but until then it might be smart to detach from that library.

[vicchi]

@massi1008 After an admittedly cursory look, it would appear that Pendulum is used to parse and validate datetime.date and datetime.datetime objects during JSON encoding. It should, in principle, be possible to swap out Pendulum for python-dateutil's parsers.