Add dev requirements file

Author: HardNorth

requirements-dev.txt

@@ -0,0 +1,3 @@
+delayed-assert
+pytest
+pytest-cov

tox.ini

@@ -11,9 +11,7 @@ envlist =
 [testenv]
 deps =
     -rrequirements.txt
-    delayed-assert
-    pytest
-    pytest-cov
+    -rrequirements-dev.txt
 
 setenv   =
     AGENT_NO_ANALYTICS = 1
@@ -25,14 +23,6 @@ skip_install = True
 deps = pre-commit>=1.11.0
 commands = pre-commit run --all-files --show-diff-on-failure
 
-[testenv:py27]
-deps =
-    -rrequirements.txt
-    delayed-assert
-    mock
-    pytest
-    pytest-cov
-
 [gh-actions]
 python =
     3.7: pep, py37