Don't install enum34 when using Python 3.4 or later. Fix #5456. (#5846)

The re stdlib module requires attrs that don't exist in the backported 3.4 version.

Technically, we are changing our install behavior beyond what is necessary. Previously, enum34 was used for 3.4 and 3.5 as well, and it happened not to conflict, but I think it's better to use the latest bug-fixed stdlib versions as long as they meet the needs of `cryptography`, which is what depends on enum34. That way, at least the various stdlib modules are guaranteed not to conflict with each other.

letsencrypt-auto-source/letsencrypt-auto

@@ -1089,7 +1089,7 @@ cryptography==2.0.2 \
     --hash=sha256:01e6e60654df64cca53733cda39446d67100c819c181d403afb120e0d2a71e1b \
     --hash=sha256:d46f4e5d455cb5563685c52ef212696f0a6cc1ea627603218eabbd8a095291d8 \
     --hash=sha256:3780b2663ee7ebb37cb83263326e3cd7f8b2ea439c448539d4b87de12c8d06ab
-enum34==1.1.2 \
+enum34==1.1.2 ; python_version < '3.4' \
     --hash=sha256:2475d7fcddf5951e92ff546972758802de5260bf409319a9f1934e6bbc8b1dc7 \
     --hash=sha256:35907defb0f992b75ab7788f65fedc1cf20ffa22688e0e6f6f12afc06b3ea501
 funcsigs==1.0.2 \

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

@@ -93,7 +93,7 @@ cryptography==2.0.2 \
     --hash=sha256:01e6e60654df64cca53733cda39446d67100c819c181d403afb120e0d2a71e1b \
     --hash=sha256:d46f4e5d455cb5563685c52ef212696f0a6cc1ea627603218eabbd8a095291d8 \
     --hash=sha256:3780b2663ee7ebb37cb83263326e3cd7f8b2ea439c448539d4b87de12c8d06ab
-enum34==1.1.2 \
+enum34==1.1.2 ; python_version < '3.4' \
     --hash=sha256:2475d7fcddf5951e92ff546972758802de5260bf409319a9f1934e6bbc8b1dc7 \
     --hash=sha256:35907defb0f992b75ab7788f65fedc1cf20ffa22688e0e6f6f12afc06b3ea501
 funcsigs==1.0.2 \