`uv-mode` doesn't restore exec-path, PATH and changes global state

`uv-mode` doesn't restore its state properly. it also applies its changes globally instead of activated buffers only.
see z80dev/uv-mode#8 for upstream report.

i think the approach used by [`envrc.el`](https://github.com/purcell/envrc/blob/master/envrc.el) should be preferred (in fact, using a `.envrc` file in the project root setting `VIRTUAL_ENV` and `PATH` has the same effect (+better behavior) as `uv-mode`.)