Fixes#7988. As described there, the steps involved are:
1. Update our tests so they fail due to this problem.
2. Update the keys used in the tests so they pass with the new changes.
For 1, see a [failing travis run](https://travis-ci.com/github/certbot/certbot/jobs/340710511) with the included change. And for the full output to confirm that this is what is failing, see a [run on debian 10](https://github.com/certbot/certbot/files/4692350/debian_run_log.txt).
This PR adds `rsa4096_key.pem` and `rsa4096_cert.pem`, updates the `TLS-ALPN` test to use those keys in place of the 1024-bit versions, and fixes the README in that `testdata` folder with correct instructions to generate these files.
* export PIP_NO_BINARY in pip install subshell in test_sdists.sh
* set environment variable on the line that installs most packages
* Generate 4096-bit rsa key and cert, and fix README instructions to do so.
* Update TLS_ALPN test to use 4096-bit key instead of 1024-bit key.
* Update changelog
* Older versions of Python have an error when both VIRTUAL_NO_DOWNLOAD and PIP_NO_BINARY are set, so only apply the latter at the install phase.
* Add enum34 constraint manually, since rebuild_dependencies.py seems to be broken.
* only delete key if it exists
* Check OpenSSL version before trying to set PIP_NO_BINARY
* Add comment explaining why we only set PIP_NO_BINARY at the install step
