Editor asks for π to be written in roman

They are referring to the upright version of pi. In the picture below, you can find on the left an italics version and on the right an upright one.

If you're using LaTeX, you can obtain the upright pi with the upgreek package and the command \uppi, see this Q&A on TeX SE.

More details on this convention can be found in the IUPAC Green book, which follows the ISO/IEC 80000 standard. From p. 7 of the linked document:

The overall rule is that symbols representing physical quantities or variables are italic, but symbols representing units, mathematical constants, or labels, are roman.

And from p. 8:

The symbols π (3.141 592. . .), e (2.718 281. . . , base of natural logarithms), i (square root of minus one), etc. are always roman [...]

They mean that it shouldn't be written in italics.

Basically, they weren't referring to the Roman and Greek civilizations, but to typefaces.

Quoting from the Encyclopedia Brittanica:

Roman, in printing, one of the three major typefaces in the history of Western typography (the others being italic and black letter, or Gothic) and, of those three, the face that is of the greatest importance and the widest use.

So, basically, by specifying that pi should be roman, they were specifying that it should be written using a particular sort of typeface, without using italics. Some of the other answers have talked about how to do so using LaTeX, but since none of them have explicitly stated what "roman" means in this context, I figured I'd write an answer explaining it.

Specifically at MNRAS, they actually provide an upright (i.e., not italic) version of π in their LaTeX class. To cite the manual:

There are several options which can be added to the document class line like this:
\documentclass[option1,option2]{mnras}
The available options are:

• ...
  
• useAMS– adds support for upright Greek characters \upi,\umuand, \upartial (π,μ and ∂). Only these three are included, if you re-quire other symbols you will need to include the amsmath or amsymb packages

I believe the request for the upright π can be inferred from their General Instructions:

Mathematics
(...) Differential d, complex i, exponential e, (...) are roman (not italic).