test: move test dependencies to tox.ini files

Move test dependencies to `tox.ini` file to allow the removal of the
`requirements` directory.

Signed-off-by: Roald Nefs <info@roaldnefs.com>

Author: roaldnefs

requirements/tests.txt

@@ -7,10 +7,14 @@
 envlist = {py36,py37,py38,py39}-install,mypy,pep8
 
 [testenv]
-deps = -rrequirements/tests.txt
+deps =
+  mypy
+  pytest
+  flake8
+  responses
 commands =
-    pytest tests {posargs}
-    mypy --config-file=tox.ini transip tests
+  pytest tests {posargs}
+  mypy --config-file=tox.ini transip tests
 
 [testenv:pep8]
 commands =
@@ -42,5 +46,3 @@ commands = {[testenv:install]commands}
 
 [flake8]
 exclude = .git,.venv,.tox,dist,doc,*egg,build,
-; max-line-length = 88
-; ignore = H501,H803