Change PWP::Encoding to PWP::SingleEncoding

Hi,

`Pod::Weaver` has `SingleEncoding` plugin now (starting from 4.000). This makes `Pod::Weaver::Plugin::Encoding` an extra dependency that could be thrown away.

See [Rik's post](http://rjbs.manxome.org/rubric/entry/2021) for details.

This PR is part of my [advent quest](https://questhub.io/realm/perl/quest/547b4202070ccf750d00010e).

Cheers,
Sergey