Fix TLS-ALPN tests with newer versions of OpenSSL (#8026)

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

letsencrypt-auto-source/letsencrypt-auto

@@ -1334,7 +1334,9 @@ cryptography==2.8 \
 distro==1.4.0 \
     --hash=sha256:362dde65d846d23baee4b5c058c8586f219b5a54be1cf5fc6ff55c4578392f57 \
     --hash=sha256:eedf82a470ebe7d010f1872c17237c79ab04097948800029994fa458e52fb4b4
-enum34==1.1.6 \
+# Package enum34 needs to be explicitly limited to Python2.x, in order to avoid
+# certbot-auto failures on Python 3.6+ which enum34 doesn't support. See #5456.
+enum34==1.1.6 ; python_version < '3.4' \
     --hash=sha256:2d81cbbe0e73112bdfe6ef8576f2238f2ba27dd0d55752a776c41d38b7da2850 \
     --hash=sha256:644837f692e5f550741432dd3f223bbb9852018674981b1664e5dc339387588a \
     --hash=sha256:6bd0f6ad48ec2aa117d3d141940d484deccda84d4fcd884f5c3d93c23ecd8c79 \

letsencrypt-auto-source/pieces/dependency-requirements.txt

@@ -78,7 +78,9 @@ cryptography==2.8 \
 distro==1.4.0 \
     --hash=sha256:362dde65d846d23baee4b5c058c8586f219b5a54be1cf5fc6ff55c4578392f57 \
     --hash=sha256:eedf82a470ebe7d010f1872c17237c79ab04097948800029994fa458e52fb4b4
-enum34==1.1.6 \
+# Package enum34 needs to be explicitly limited to Python2.x, in order to avoid
+# certbot-auto failures on Python 3.6+ which enum34 doesn't support. See #5456.
+enum34==1.1.6 ; python_version < '3.4' \
     --hash=sha256:2d81cbbe0e73112bdfe6ef8576f2238f2ba27dd0d55752a776c41d38b7da2850 \
     --hash=sha256:644837f692e5f550741432dd3f223bbb9852018674981b1664e5dc339387588a \
     --hash=sha256:6bd0f6ad48ec2aa117d3d141940d484deccda84d4fcd884f5c3d93c23ecd8c79 \