Use raw strings to avoid invalid escape sequences

[evertrol]

I noticed a SyntaxWarning (not yet an error) when trying to run the tests with the Python 3.8 beta build. While 3.8 is still in beta, this does indeed appear to be a potential problem, and easy to avoid by using raw strings.

[coveralls]

Coverage remained the same at 96.707% when pulling 4df9f8f on evertrol:syntax-correction into e75b4eb on SciTools-incubator:master.

[bjlittle]

@evertrol Have you signed the CLA - It should have been automatically sent to you?

[evertrol]

I saw the notice by the checker, but I feel for such a trivial change, a CLA should not be necessary (and, generally, implicit by submitting a PR).
Also, I'm technically not allowed to sign a CLA by my employer, and I would have to get another person (or several) involved, which seems unnecessary.

[bjlittle]

Nice one, thanks @evertrol 😄