PERF: 3x speedup in Series of dicts with datetime keys by not having error message scale with input
Surprisingly, the majority of the time spent in constructing a `Series` from a dict with `datetime`-like keys is spent formatting the keys into strings for an error message that gets suppressed.

As there's a test ensuring the string case makes it into the error message, we preserve the behavior there. In non-string cases, we mirror how the other `Dtypes` handle this case and do not include the input.
```
$ asv compare v0.24.0rc1 HEAD -s
       before           after         ratio
     [fdc4db25]       [5c82eab5]
     <v0.24.0rc1^0>       <series_dict_speedup>
-      1.14±0.02s          303±4ms     0.27  series_methods.SeriesConstructor.time_constructor('dict') 
```

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
