Make Flake8 to be a dependency.

Author: trekhleb

.flake8

@@ -0,0 +1,2 @@
+[flake8]
+max-line-length = 100

.travis.yml

@@ -5,15 +5,11 @@ python:
 # Install dependencies.
 install:
   - pip install -r requirements.txt
-  - pip install flake8
 
 # Run linting and tests.
 script:
-  # stop the build if there are Python syntax errors or undefined names
-  - flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
-  # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
-  - flake8 . --count --exit-zero --max-complexity=12 --max-line-length=127 --statistics
   - pylint ./src
+  - flake8 ./src --statistics --count
   - pytest
 
 # Turn email notifications off.

README.md

@@ -174,7 +174,9 @@ pytest ./path/to/the/test_file.py
 
 ## Linting the Code
 
-Linting is done using [pylint](http://pylint.pycqa.org/) library.
+Linting is done using [pylint](http://pylint.pycqa.org/) and [flake8](http://flake8.pycqa.org/en/latest/) libraries.
+
+### PyLint
 
 To check if the code is written with respect
 to [PEP 8](https://www.python.org/dev/peps/pep-0008/) style guide please run:
@@ -189,3 +191,22 @@ specific error by running:
 ```bash
 pylint --help-msg=missing-docstring
 ```
+
+[More about PyLint](http://pylint.pycqa.org/)
+
+### Flake8
+
+To check if the code is written with respect
+to [PEP 8](https://www.python.org/dev/peps/pep-0008/) style guide please run:
+
+```bash
+flake8 ./src
+```
+
+Or if you want to have more detailed output you may run:
+
+```bash
+flake8 ./src --statistics --show-source --count
+```
+
+[More about Flake8](http://flake8.pycqa.org/en/latest/)

requirements.txt

@@ -1,13 +1,16 @@
 astroid==2.0.4
 atomicwrites==1.1.5
 attrs==18.1.0
+flake8==3.5.0
 isort==4.3.4
 lazy-object-proxy==1.3.1
 mccabe==0.6.1
 more-itertools==4.3.0
 pluggy==0.7.1
 py==1.5.4
+pycodestyle==2.3.1
+pyflakes==1.6.0
 pylint==2.1.1
 pytest==3.7.2
 six==1.11.0
-wrapt==1.10.11
+wrapt==1.10.11

src/modules/test_modules.py

@@ -39,7 +39,6 @@
 # It can also be used when utilising from with similar effects:
 from fibonacci_module import fibonacci_at_position as fibonacci_at_position_renamed
 
-
 # When a module named spam is imported, the interpreter first searches for a built-in module with
 # that name. If not found, it then searches for a file named spam.py in a list of directories
 # given by the variable sys.path. sys.path is initialized from these locations:
@@ -48,6 +47,7 @@
 # - PYTHONPATH (a list of directory names, with the same syntax as the shell variable PATH).
 # - The installation-dependent default.
 
+
 def test_modules():
     """Modules"""