BUG: make parser.pyx method polymorphic for gil/nogil cases
- [x] closes #27372 
- [ ] Cython 0.30 released

**Warning**: this requires Cython 0.30 (not yet released as of Jul 2019) (the CI currently fails to build)

The [next release](https://cython.readthedocs.io/en/latest/src/changes.html#features-added) of Cython 0.30.0 [introduced](https://github.com/cython/cython/issues/2579) conditional gil acqusition with support for fused types.

This makes a clean solution for #27372 possible, eliminating the cython warning
```
warning: pandas/_libs/parsers.pyx:1724:34: Casting a GIL-requiring 
function into a nogil function circumvents GIL validation
```
