Switch from coveralls to codecov for Travis CI

Replicates graphql/graphql-js@4ed25af

One of the issues for us with coveralls was this:
z4r/python-coveralls#66

Author: Cito

.coveragerc

@@ -0,0 +1,12 @@
+[run]
+branch = True
+source = graphql
+
+[report]
+exclude_lines =
+    pragma: no cover
+    raise NotImplementedError
+ignore_errors = True
+omit =
+docs/*
+tests/*

.travis.yml

@@ -22,4 +22,4 @@ script:
     - pytest --cov-report term-missing --cov=graphql
 
 after_success:
-    - coveralls
+    - codecov

Pipfile

@@ -14,8 +14,8 @@ pytest-asyncio = "*"
 tox = "*"
 sphinx = "*"
 sphinx_rtd_theme = "*"
-python-coveralls = "*"
-pytest-cov = ">=2.4,<2.6"
+codecov = "*"
+pytest-cov = "*"
 pyyaml = ">=4.0"
 
 [pipenv]

Pipfile.lock

@@ -7,7 +7,7 @@ a query language for APIs created by Facebook.
 [![PyPI version](https://badge.fury.io/py/GraphQL-core-next.svg)](https://badge.fury.io/py/GraphQL-core-next)
 [![Documentation Status](https://readthedocs.org/projects/graphql-core-next/badge/)](https://graphql-core-next.readthedocs.io)
 [![Build Status](https://travis-ci.com/graphql-python/graphql-core-next.svg?branch=master)](https://travis-ci.com/graphql-python/graphql-core-next)
-[![Coverage Status](https://coveralls.io/repos/github/graphql-python/graphql-core-next/badge.svg?branch=master)](https://coveralls.io/github/graphql-python/graphql-core-next?branch=master)
+[![Coverage Status](https://codecov.io/gh/graphql-python/graphql-core-next/branch/master/graph/badge.svg)](https://codecov.io/gh/graphql-python/graphql-core-next)
 [![Dependency Updates](https://pyup.io/repos/github/graphql-python/graphql-core-next/shield.svg)](https://pyup.io/repos/github/graphql-python/graphql-core-next/)
 [![Python 3 Status](https://pyup.io/repos/github/graphql-python/graphql-core-next/python-3-shield.svg)](https://pyup.io/repos/github/graphql-python/graphql-core-next/)
 [![Code Style](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/ambv/black)

README.md

@@ -41,7 +41,7 @@
         "flake8",
         "mypy",
         "tox",
-        "python-coveralls",
+        "codecov",
     ],
     packages=find_packages(include=["graphql"]),
     include_package_data=True,