setup.py: Add test dependencies in external file

Closes #222

.travis.yml

@@ -33,7 +33,7 @@ before_install:
 
 install:
   - pip install -e .;
-  - pip install pytest;
+  - pip install -r requirements-tests.txt;
 
 script:
   - pytest

appveyor.yml

@@ -9,7 +9,7 @@ environment:
 
 install:
   -  '%PYTHON%\Scripts\pip install -e .'
-  -  '%PYTHON%\Scripts\pip install pytest'
+  -  '%PYTHON%\Scripts\pip install -r requirements-tests.txt'
 build: off
 test_script:
   - '%PYTHON%\Scripts\pytest'

requirements-tests.txt

@@ -0,0 +1 @@
+pytest

setup.py

@@ -5,15 +5,21 @@ def readme():
     with open('README.md') as file:
         return file.read()
 
-def requirements():
-    with open('requirements.txt') as file:
+def requirements(section=None):
+    if section is None:
+        filename = "requirements.txt"
+    else:
+        filename = f"requirements-{section}.txt"
+
+    with open(filename) as file:
         return [line.strip() for line in file]
 
 setup(
     name='ppb',
     version='0.5.0',
     packages=['ppb', 'ppb.systems'],
     install_requires=requirements(),
+    tests_require=requirements('tests'),
     python_requires=">=3.6",
     url='https://github.com/ppb/pursuedpybear',
     license='Artistic-2.0',