Cython warning about unsafe nogil cast in parsers.pyx
```
commit 681e6a9b07271a0955e6780e476ab2d7101e549c
Author: Phil Ruffwind <rf@rufflewind.com> 

    BUG: Segfault due to float_precision='round_trip'
    
    `round_trip` calls back into Python, so the GIL must be held.  It also
    fails to silence the Python exception, leading to spurious errors.
    Closes #15140. 
```    

681e6a9b07271a0955e6 closed #15140 but introduced a cython warning (seen with 0.29.12)
```
warning: pandas/_libs/parsers.pyx:1724:34: Casting a GIL-requiring 
function into a nogil function circumvents GIL validation
```

The commit says it fixes a segfault, while the cython warning suggests this is still unsafe.

The lines in question are https://github.com/pandas-dev/pandas/blob/8913b7e84ac75b1093bf708e0d1a9a0789e8cfcd/pandas/_libs/parsers.pyx#L1723-L1727

