chore: Python 3.9.0 final, Python 3.7 constraints (#440)

* chore: update to 3.9.0 final

* chore: add python 3.7 constraints

* chore: missed one image due to travis-ci fail

Co-authored-by: Matthieu Darbois <mayeut@users.noreply.github.com>

Co-authored-by: Matthieu Darbois <mayeut@users.noreply.github.com>

docs/contributing.md

@@ -38,29 +38,69 @@ cibuildwheel's _minimal_ example configs can be tested on a simple project on ci
 
 To test minimal configs, make sure you have a clean git repo, then run the script:
 
-    bin/run_example_ci_configs.py
+```bash
+bin/run_example_ci_configs.py
+```
 
 The script will create an isolated 'orphan' commit containing all the minimal config CI files, and a simple C extension project, and push that to a branch on the `origin` repo. The project's CI is already set up to run on branch push, so will begin testing.
 
 The script then outputs a Markdown table that can be copy/pasted into a PR to monitor and record the test.
 
+## Preparing environments
+
+The dependency update script in the next section requires multiple python versions installed. One way to do this is to use `pyenv`:
+
+```bash
+pyenv install 2.7.18
+pyenv install 3.5.9
+pyenv install 3.6.11
+pyenv install 3.7.8
+# Optionally add 3.8 and make it the local version;
+# otherwise assuming 3.8+ already is your current python version
+```
+
+Then, you need to make the required virtual environments:
+
+```bash
+$(pyenv prefix 2.7.18)/bin/python -m pip install virtualenv
+$(pyenv prefix 2.7.18)/bin/python -m virtualenv env27
+$(pyenv prefix 3.5.9)/bin/python -m venv env35
+$(pyenv prefix 3.6.11)/bin/python -m venv env36
+$(pyenv prefix 3.7.8)/bin/python -m venv env37
+```
+
+<!-- Note for fish users: use zsh/bash for these lines for now, there's not a nice one-line fish replacement -->
+
+And, you need to install the requirements into each environment:
+
+```bash
+for f in env*/bin/pip; do $f install pip-tools; done
+```
+
+
 ## Making a release
 
 Before making a release, ensure pinned dependencies are up-to-date. Run the script:
 
-    bin/make_dependency_update_pr.py
+```bash
+bin/make_dependency_update_pr.py
+```
 
 If updates are needed, this will push a PR with those updates for the CI to test. Once green, merge this PR.
 
 Then, increment the project version number using:
 
-    bin/bump_version.py
+```bash
+bin/bump_version.py
+```
 
 You'll be prompted to enter the new version number. Update the changelog when prompted. The script will create a 'bump version' commit and version tag.
 
 Finally, cut the release and upload to PyPI/Github.
 
-    rm -rf dist
-    python setup.py sdist bdist_wheel
-    twine upload dist/*
-    git push && git push --tags
+```bash
+rm -rf dist
+python setup.py sdist bdist_wheel
+twine upload dist/*
+git push && git push --tags
+```