Timex.format seems off at the end of the month

### Steps to reproduce

```
iex> in_5_months = Timex.shift(Timex.now(), months: 5)
~U[2023-06-30 07:51:27.003055Z]
iex> Timex.format!(in_5_months, "{relative}", :relative)
"in 4 months"
```

### Description of issue

- Timex.format seems to miss a month - the result should be "in 5 months", instead of "in 4 months". The output of Timex.shift looks good.
- It seems the error is only occuring starting recently, it was fine before.
- At first I thought it was because due to me testing on the 31st. But apparently the issue is not related to the date. I guess it depends if Timex.format does some rounding? Otherwise 5 months - a few secons is already 4 months. It works fine if I add another hour. I'm just curious why it worked before.

Could this be related to this issue? https://github.com/bitwalker/timex/issues/535

I hope I didn't miss any other open issue related to this. Thank you for the great work!

Start by reproducing the reported Timex.shift and Timex.format! calls using the "{relative}" formatter and :relative option, then inspect the relative-formatting entry point. Compare the result near month boundaries and with an added hour. Done means the example reports the expected month count and regression coverage demonstrates the corrected behavior.

Written by the indexing model from the issue text.