Replace git.openstack.org URLs with opendev.org URLs

1. Replace git.openstack.org URLs with opendev.org URLs
2. bump tox minversion to 2.0 to keep consistence with magmum[1]

[1]: https://github.com/openstack/magnum/blob/master/tox.ini#L2

Change-Id: Iddc8ca8de4451b7a152f5526ff9f25794dde734e
Closes-Bug: #1826698

CONTRIBUTING.rst

@@ -13,4 +13,4 @@ Pull requests submitted through GitHub will be ignored.
 
 Bugs should be filed on Launchpad, not GitHub:
 
-   https://bugs.launchpad.net/python-magnumclient
+   https://bugs.launchpad.net/python-magnumclient

README.rst

@@ -37,4 +37,4 @@ in the `developer guide
 .. _Online Documentation: https://docs.openstack.org/python-magnumclient/latest/
 .. _Launchpad project: https://launchpad.net/python-magnumclient
 .. _Bugs: https://bugs.launchpad.net/python-magnumclient
-.. _Source: https://git.openstack.org/cgit/openstack/python-magnumclient
+.. _Source: https://opendev.org/openstack/python-magnumclient

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 1.6
+minversion = 2.0
 envlist = py27,py37,pypy,pep8
 skipsdist = True
 
@@ -11,7 +11,7 @@ setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
 deps =
-       -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+       -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
@@ -21,14 +21,14 @@ commands =
 [testenv:bandit]
 basepython = python3
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
   -r{toxinidir}/test-requirements.txt
 commands = bandit -r magnumclient -x tests -n5 -ll
 
 [testenv:pypy]
 basepython = python3
 deps =
-       -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+       -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
        setuptools<3.2
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt