Timezone MDT not supported for parsing (yet it works for formatting)

### Steps to reproduce

```
format_str = "{YYYY}-{M}-{D} {h24}:{m}:{s} {Zabbr}"
{:ok, datetime} = DateTime.new(~D[2021-09-13], ~T[00:00:00.000], "America/Denver")
{:ok, d } = Timex.format(datetime, format_str) # this outputs MDT as the timezone, but it cannot parse the datetime back from that string.
IO.puts d # => "2021-9-13 00:00:00 MDT"
IO.inspect Timex.parse(d, format_str) # => {:error, :time_zone_not_found}
```

### Description of issue

- Using the same input string, the formatted output should be able to be re-parsed to a date time.
- Clearly the library recognizes MDT as a valid date time because I do not have MDT anywhere in my code. It is correctly converting from America/Denver to MDT, but it cannot parse it back from MDT again.
- In the above example, the output of d should match the input of datetime. I believe the format call will return MDT part of the year and MST part of the year, but the end result or parsing it again should still return the same DateTime