Move development reqirements to dev-requirements.txt

Description

This is backport of !637 (merged) to bst-1.2.

There were a couple of differences between the requirements listed in the master branch and the bst-1.2 branch so I have left them untouched.

For reference, here are the differences between them:

--- a/dev-requirements.txt
+++ b/dev-requirements.txt
@@ -2,8 +2,9 @@
 # random crashes with 4.4.2
 coverage == 4.4.0
 pep8
+pylint >= 1.8 , < 2
 pytest >= 3.1.0
-pytest-cov >= 2.5.0
+pytest-cov
 pytest-datafiles
 pytest-env
 pytest-pep8

Changes proposed in this merge request:

• Move development reqirements to dev-requirements.txt

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