Taking over maintainership of exact-pi

[Andrew Lelechenko]

I would like to take over `exact-pi` package (https://hackage.haskell.org/package/exact-pi).
Unfortunately, the package became incompatible with upcoming GHC 8.4 and needs an upgrade. The relevant pull request (https://github.com/dmcclean/exact-pi/pull/5), prepared by @konn, has been open for a month without response. I also tried to reach its maintainer Douglas McClean (cced) by email, but have not heard back.

I am so interested in upgrading `exact-pi`, because it is a dependency of my package `arithmoi`.

— 
Best regards,
Andrew