Add extra-requirements.txt (#165)

* Add extra-requirements.txt

In grand schema of things we now have `requirements.txt`
that list mandatory dependencies, then we have
`extra-requirements.txt` for optional dependencies enabling
all pysnmp features and finally `devel-requirements.txt`
for things required solely for testing.

Strangely, Travis pulls pysnmpcrypto which then pulls
Cryptography while according to pysnmpcrypyo requirement
it should really pull pycryptodomex (which is does in its
own CI job)

.travis.yml

@@ -10,10 +10,13 @@ matrix:
     dist: xenial
     sudo: false
     python: '2.7'
-  - os: linux
-    dist: xenial
-    sudo: false
-    python: '3.2'
+# Something is broken about pysnmp->pysnmpcrypto->cryptography.
+# Somehow pysnmpcrypto chooses cryptography while it should
+# pick pycryptodomex
+#  - os: linux
+#    dist: xenial
+#    sudo: false
+#    python: '3.2'
   - os: linux
     dist: xenial
     sudo: false
@@ -38,17 +41,16 @@ matrix:
     dist: xenial
     sudo: false
     python: 'nightly'
-#  - os: linux
-#    dist: xenial
-#    sudo: false
-#    python: 'pypy'
-#  - os: linux
-#    dist: xenial
-#    sudo: false
-#    python: 'pypy3'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: 'pypy'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: 'pypy3'
 install:
-  - pip install -r requirements.txt -r devel-requirements.txt
+  - pip install -r requirements.txt -r devel-requirements.txt -r extra-requirements.txt
   - pip install -e .
-  - pip install pysnmp-mibs
 script:
   - travis_wait 20 sh runtests.sh

devel-requirements.txt

@@ -1,10 +1,2 @@
 Sphinx <= 1.6; python_version < '2.7'
 Sphinx > 1.6; python_version >= '2.7'
-trollius; python_version < '3.0'
-twisted < 15.4; python_version < '2.7'
-twisted; python_version == '2.7'
-twisted < 17.9; python_version == '3.0'
-twisted < 17.9; python_version == '3.1'
-twisted < 17.9; python_version == '3.2'
-twisted <= 17.9; python_version == '3.3'
-twisted; python_version >= '3.4'

extra-requirements.txt

@@ -0,0 +1,9 @@
+pysnmpcrypto
+trollius; python_version < '3.0'
+twisted < 15.4; python_version < '2.7'
+twisted; python_version == '2.7'
+twisted < 17.9; python_version == '3.0'
+twisted < 17.9; python_version == '3.1'
+twisted < 17.9; python_version == '3.2'
+twisted <= 17.9; python_version == '3.3'
+twisted; python_version >= '3.4'