CLN: different way of doing nogil blocks in templates?
Right now we have blocks in the `pxi.in` files that look this, which are somewhat error prone to edit/write.

``` python
{{nogil_str}}
{{tab}}prev = arr[0]
{{tab}}for i in range(1, n):
{{tab}}    cur = arr[i]
{{tab}}    if timelike and cur == iNaT:
{{tab}}        is_monotonic_inc = 0
{{tab}}        is_monotonic_dec = 0
```

This is the best alternative I could come up with, where `maybe_nogil` is a formatting function; `tempita` uses `|` as a pipe operator.  It's somewhat annoying to look at in that the whole block gets syntax highlighted as a string, but may be cleaner and plays nicer with an editor than what's being done now - thoughts? cc @sinhrks 

``` python
{{'''
prev = arr[0]
for i in range(1, n):
    cur = arr[i]
    if timelike and cur == iNaT:
        is_monotonic_inc = 0
        is_monotonic_dec = 0
''' | maybe_nogil }}}



```
