Add support for gitpod
This PR proposes to add support for Gitpod, which can provide (new) contributors with one-click environment to use for working on the pandas code base. 

The PR includes two main additions:
- .gitpod.yml - Config file for gitpod that specifies which image/Dockerfile to use for gitpod and which init command(s) to run when booting the pod.
- .gitpod.Dockerfile - Dockerfile used to build a base image for the pod. Partially adapted from the existing Dockerfile in the repo.

Some things maybe still to consider:
- Gitpod has some options for providing pre-built workspaces (https://www.gitpod.io/docs/prebuilds/). This may be interesting to enable.
- There is some duplication between the two dockerfiles, but unfortunately the VS Code Dockerfile is not entirely compatible with the gitpod one. As such, the duplication may not be a bad thing. 

- [x] closes #xxxx (N/A)
- [x] tests added / passed (N/A)
- [x] passes `black pandas` (N/A)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff` (N/A)
- [x] whatsnew entry (N/A)
