Use Python 3.9 as the default Python version for new apps

Previously if a new app did not specify a Python version via a
`runtime.txt` or the appropriate `Pipfile` config option, then
the buildpack would default to Python 3.6.13.

Now the buildpack will use Python 3.9.2.

This does not affect:
* apps that specify an explicit Python version
* existing apps that have already had a build using
  the previous default version (since that version is
  remembered from one build to the next, via the
  "sticky versions" feature).

Use-cases involving new apps that aren't compatible with
Python 3.9 will now need to specify Python 3.6 explicitly:
https://devcenter.heroku.com/articles/python-support#specifying-a-python-version

Closes GUS-W-7686321.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Use Python 3.9.2 as the default Python version for new apps (previously Python 3.6.13) ([#1187](https://github.com/heroku/heroku-buildpack-python/pull/1187)).
 - Remove Airflow `SLUGIFY_USES_TEXT_UNIDECODE` workaround ([#1186](https://github.com/heroku/heroku-buildpack-python/pull/1186)).
 - Fix grammar in the Python 2 EOL message ([#1182](https://github.com/heroku/heroku-buildpack-python/pull/1182)).
 

README.md

@@ -42,7 +42,7 @@ A `requirements.txt` must be present at the root of your application's repositor
 
 To specify your python version, you also need a `runtime.txt` file - unless you are using the default Python runtime version.
 
-Current default Python Runtime: Python 3.6.12
+Current default Python Runtime: Python 3.9.2
 
 Alternatively, you can provide a `setup.py` file, or a `Pipfile`.
 Using `pipenv` will generate `runtime.txt` at build time if one of the field `python_version` or `python_full_version` is specified in the `requires` section of your `Pipfile`.

bin/default_pythons

@@ -14,4 +14,4 @@ LATEST_34="python-3.4.10"
 LATEST_27="python-2.7.18"
 LATEST_PYPY_36="pypy3.6-7.3.2"
 LATEST_PYPY_27="pypy2.7-7.3.2"
-DEFAULT_PYTHON_VERSION="${LATEST_36}"
+DEFAULT_PYTHON_VERSION="${LATEST_39}"

spec/hatchet/nltk_spec.rb

@@ -11,7 +11,7 @@
         expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
           remote: -----> Downloading NLTK corpora…
           remote: -----> Downloading NLTK packages: city_database stopwords
-          remote: /app/.heroku/python/lib/python3.6/runpy.py:125: RuntimeWarning: 'nltk.downloader' found in sys.modules after import of package 'nltk', but prior to execution of 'nltk.downloader'; this may result in unpredictable behaviour
+          remote: .*: RuntimeWarning: 'nltk.downloader' found in sys.modules after import of package 'nltk', but prior to execution of 'nltk.downloader'; this may result in unpredictable behaviour
           remote:   warn\\(RuntimeWarning\\(msg\\)\\)
           remote: \\[nltk_data\\] Downloading package city_database to
           remote: \\[nltk_data\\]     /tmp/build_.*/.heroku/python/nltk_data...

spec/spec_helper.rb

@@ -15,7 +15,7 @@
 LATEST_PYTHON_3_9 = '3.9.2'
 LATEST_PYPY_2_7 = '7.3.2'
 LATEST_PYPY_3_6 = '7.3.2'
-DEFAULT_PYTHON_VERSION = LATEST_PYTHON_3_6
+DEFAULT_PYTHON_VERSION = LATEST_PYTHON_3_9
 
 # Work around the return value for `default_buildpack` changing after deploy:
 # https://github.com/heroku/hatchet/issues/180