Enable isolated_build for tox

This enables tox support for PEP517 and PEP518.

MANIFEST.in

@@ -1,5 +1,6 @@
 include README.rst
 include LICENSE.rst
+include pyproject.toml
 recursive-include requirements *
 recursive-include src/drf_yasg/static *
 recursive-include src/drf_yasg/templates *

pyproject.toml

@@ -1,2 +1,3 @@
 [build-system]
 requires = ["setuptools", "wheel", "setuptools-scm"]
+build-backend = "setuptools.build_meta"

requirements/tox.txt

@@ -1,2 +1,2 @@
 # requirements for building and running tox
-tox>=3.1.2
+tox>=3.3.0

tox.ini

@@ -1,11 +1,18 @@
 [tox]
 # https://docs.djangoproject.com/en/dev/faq/install/#what-python-version-can-i-use-with-django
+minversion = 3.3.0
+isolated_build = true
+isolated_build_env=.package
 envlist =
     py{27,34,35,36}-django111-drf{37,38,39},
     py{34,35,36,37}-django20-drf{37,38,39},
     py{35,36,37}-django21-drf{37,38,39},
     djmaster, lint, docs
 
+[testenv:.package]
+# no additional dependencies besides PEP 517
+deps =
+
 [testenv]
 deps =
     django111: Django>=1.11,<2.0
@@ -21,8 +28,8 @@ deps =
     djmaster: https://github.com/django/django/archive/master.tar.gz
 
     # other dependencies
-    -rrequirements/validation.txt
+    -r requirements/validation.txt
-    -rrequirements/test.txt
+    -r requirements/test.txt
 
 commands =
     pytest -n 2 --cov --cov-config .coveragerc --cov-append --cov-report="" {posargs}
@@ -30,13 +37,13 @@ commands =
 [testenv:lint]
 skip_install = true
 deps =
-    -rrequirements/lint.txt
+    -r requirements/lint.txt
 commands =
     flake8 src/drf_yasg testproj tests setup.py
 
 [testenv:docs]
 deps =
-    -rrequirements/docs.txt
+    -r requirements/docs.txt
 commands =
     python setup.py check --restructuredtext --metadata --strict
     sphinx-build -WnEa -b html docs docs/_build/html