Drop python2 support within test suite (tox and travis)

Ref,; #233

Author: elbenfreund

.travis.yml

@@ -7,7 +7,7 @@ language:
   - python
 
 python:
-  - "2.7"
+  - "3.7"
 
 services:
   - docker

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 # We skip isort because it conflicts with setting GTK version in inports
 # https://github.com/timothycrosley/isort/issues/295
-envlist = py27, py35, docs, flake8, manifest, pep257
+envlist = py37, docs, flake8, manifest, pep257
 
 [testenv]
 setenv =
@@ -19,10 +19,10 @@ commands =
     make coverage
 
 [testenv:docs]
-basepython = python2
+basepython = python3
 # We need access to GTK for autodocs
 sitepackages = True
-deps = doc8==0.7.0
+deps = doc8
 commands =
     pip install -r requirements/docs.pip
     make docs BUILDDIR={envtmpdir} SPHINXOPTS={env:SPHINXOPTS_BUILD:''}
@@ -32,24 +32,22 @@ commands =
 [testenv:flake8]
 basepython = python3
 deps =
-    flake8==3.2.1
-    flake8-debugger==1.4.0
-    flake8-print==2.0.2
-    pep8-naming==0.4.1
+    flake8
+    flake8-debugger
+    flake8-print
+    pep8-naming
 skip_install = True
 commands = flake8 setup.py hamster_gtk/ tests/
 
 [testenv:manifest]
 basepython = python3
-deps = check-manifest==0.35
+deps = check-manifest
 skip_install = True
 commands =
     check-manifest -v
 
 [testenv:pep257]
 basepython = python3
 skip_install = True
-deps =
-    pep257==0.7.0
-commands =
-    pep257 setup.py hamster_gtk/ tests/
+deps = pep257
+commands = pep257 setup.py hamster_gtk/ tests/