PERF: regression fixup for timestamp.TimestampConstruction.time_parse_iso8601_tz benchmark
- [x] closes #N/A
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

For detection details see: https://github.com/pandas-dev/pandas/pull/26220#issuecomment-491143978

The slowdown was due to the use of `convert_to_tsobject` function.

Asv result:

master| patch| ratio| test_name
-|-|-|-
31.3±0.5μs|       21.9±0.2μs|     0.70| timestamp.TimestampConstruction.time_parse_iso8601_tz


