This ps adds py39 support and replaces dashes with underscores

Change-Id: I5245be419244c8bd93c7099b61ce0a382f7a5065

This PR replaces a request parameter name "net-id" with "network_id"
creating a cluster. When creating a cluster, Clusters API internally
calls Instaces API to create a instance. A request parameter name to
create instances has changed[1] but python-troveclient still uses
old parameter name when creating a cluster.

The original problem is instance creation error when creating a
instance that belongs to a cluster. Troves's Instances API has
changed a parameter name from "net-id" to "network_id", but Clusters
API and python-troveclient still uses the "net-id" parameter name.
We need to fix it.

[1]: https://opendev.org/openstack/trove/commit/535417240709694080255c064c9d84a5ab1d5372

Task: 44925
Story: 2009958
Change-Id: Ia3d1d6f451296eba34f2389da184c41a7ee9e84e

This PR makes troveclient to handle API response in new data schema.
Trove API response data schema has changed by the commit[1].

Original problem is that python-troveclient can't parse a API response
data in new data schema, resulting in an error.

[1]: https://opendev.org/openstack/trove/commit/429c39890e3242c6a502037673943b38452c5811

Task: 44986
Story: 2009979
Change-Id: I2e446c68c3b82c11d13f6bace54273c109e02069

Add file to the reno documentation build to show release notes for
stable/yoga.

Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/yoga.

Sem-Ver: feature
Change-Id: I8143ca09cafc3bf2ae68c856f426f2acaa699b38

This is an automatically generated patch to ensure unit testing
is in place for all the of the tested runtimes for yoga.

See also the PTI in governance [1].

[1]: https://governance.openstack.org/tc/reference/project-testing-interface.html

Change-Id: Ibb1f0b5a7740d36fda7be637f21802e31623baba

Add file to the reno documentation build to show release notes for
stable/xena.

Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/xena.

Sem-Ver: feature
Change-Id: I62ae17b426c970e0e595210da0d7656f70b37352

Change-Id: Id0853dfb207bbd219755d46a9c0d6b4e897bfbeb

The patch bumps min version of tox to 3.18.0 in order to
replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: Ia9928d8fe69a6a3c593ff9b5846ef3123e7dda26

Change-Id: I97d3fcfa2935f76cb8753734fec6dab8669e732a