Upgrade Certbot dependencies (certbot#8036)

This PR upgrades Certbot pinned dependencies through `letsencrypt-auto-source/rebuild_dependencies.py` while taking into account the problems detected in certbot#8035:
* `cryptography` is pinned to `2.8` to continue to support OpenSSL 1.0.1 on non-x86 ancient Linux distributions (RHEL 6 + Debian 8)
* `parsedatetime` is pinned to `2.5` because of an incompatibility with Python 2.7 (see bear/parsedatetime#246)
* `letsencrypt-auto-source/rebuild_dependencies.py` now takes into account the environment markers that are aded to `AUTHORITATIVE_CONSTRAINTS`: this is used for the `enum34` dependency, to not install it on Python 3.6+ and not break the distribution by swapping the built-in `enum` module during the setup of Certbot venv.

Fixes certbot#8030

* Pin cryptography and parsedatetime

* Upgrade dependencies

* Remove authoritative constraint

* Upgrade dependencies

* Rebuild certbot-auto

* Update letsencrypt-auto-source/rebuild_dependencies.py

Co-authored-by: Brad Warren <[email protected]>

* Honor specific requirements in the AUTHORITATIVE_CONSTRAINTS

* Fix injection

* Update dependencies

* Update rebuild_dependencies.py

Co-authored-by: Brad Warren <[email protected]>

Author: adferrand