TLS cert for dev.pandas.io is broken

http is a security risk now days and when using a custom domain for github pages, you can't rely on github's wildcard cert for https, since you get a warning.

Mozilla's (let's encrypt)[https://letsencrypt.org] makes it easy to get a free cert in 5 minutes.

cc @datapythonista,

[datapythonista]

GitHub should generate the certificate automatically for us, but it's not yet ready, we've got a Not yet available for your site because the certificate has not finished being issued..

[datapythonista]

Just messaged GitHub support with the problem. This is the message I sent, will update with the answer.

We added the custom domain dev.pandas.io to https://github.com/pandas-dev/pandas-dev.github.io/settings github pages, but after few days we still see the error:

"""
Enforce HTTPS — Not yet available for your site because the certificate has not finished being issued.
Please allow 24 hours for this process to complete. (dev.pandas.io)

HTTPS provides a layer of encryption that prevents others from snooping on or tampering with traffic to your site.
When HTTPS is enforced, your site will only be served over HTTPS. Learn more. 
"""

We are replacing the content of the repo with every new version of our documentation (few times a day). We push the content of the repo with a `git push -f`, which means that we also recreate the `CNAME` file for the custom domain several times a day. Not sure if it can be related.

I understand that once the certificate is created should be available, even if we may be automatically unsetting and setting the custom domain.

Can you please assist? Thank you!

[datapythonista]

Fixed.