BUG: make parser.pyx method polymorphic for gil/nogil cases

[ghost]

• closes Cython warning about unsafe nogil cast in parsers.pyx #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 of Cython 0.30.0 introduced 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

[jreback]

this is highly unlikely to be looked at until cython is out for a while

[ghost]

this is highly unlikely to be looked at until cython is out for a while

Of course.

[ghost]

@jbrockmendel, I'm clearing my desk of pandas PRs. This looks like it would be useful in the future, would you like to grab a local copy and take over or should I just delete?

[jbrockmendel]

@pilkibun is leaving this on the back-burner until cython 30 is out (plus some time for possible bugfix release)?

[ghost]

@jbrockmendel moving on to other interests and would like to to leave a clean slate. Best if you pull the branch and push it at your convenience sometime in the future. If you're not interested that's fine too of course.

[ghost]

Ok, someone will take care of this eventually.