Review/Cleanup of master for release prep (zappa#1147)

* 🔧 update for case where expected metadata for given package does not contain 'releases'

* :penicl: add warning for case where expected `package_version` is not found.

* 🔧 change pipenv req to version greater than previously reported problem version.

* 🙈 Add `Pipfile.lock` to `.gitignore` (When supporting multiple versions of python with pipenv don't include the lock file.

* 🔧 fix pipenv version restriction.

* 🎨 run black on `core.py`

* 🎨 fix flake8, run black

* 🔧 adjust caching (requirements.txt no longer used.

* 🐛 fix Pipfile typo

* 🔥 The python3.6 runtime is deprecated by aws (July 18, 2022 [phase-1], Aug 17, 2022 [phase-2]) remove from tests for next release candidate

* 🎨 run black
🔧 update black check command.

* 🎨 run isort

* 📝 change version 0.54.1 -> 0.55.1
🔥 remove 3.6 from SUPPORTED_VERSIONS

* 🔥 remove 3.6 support

* 🔀 merge with `0.54.2-release` branch to apply 3.9 support documentation updates.
🔥 remove 3.6 support from README

* 🔀 merge with `0.54.2-release` branch to apply 3.9 support documentation updates.

* ⏪️revert __version__ back to available pypi version

Without a pypi available release the `test_slim_handler` testcase fails.

```
ERROR: No matching distribution found for zappa==0.55.1
E...................
======================================================================
ERROR: test_slim_handler (tests.tests.TestZappa)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/Zappa/Zappa/tests/tests.py", line 2211, in test_slim_handler
    zappa_cli.create_package()
  File "/home/runner/work/Zappa/Zappa/zappa/cli.py", line 2430, in create_package
    venv=self.zappa.create_handler_venv(),
  File "/home/runner/work/Zappa/Zappa/zappa/core.py", line 494, in create_handler_venv
    raise EnvironmentError("Pypi lookup failed")
OSError: Pypi lookup failed
-------------------- >> begin captured stdout << ---------------------
```

* 📝 update CHANGELOG with committed changes since last release (5b6e241)

* 🔥 remove 3.6 from docs

* 🔧 address review comments

* ✨ change release to 0.55.1

* ♻️ Update to allow specification of the latest pypi version for `test_slim_handler()`

* ✅ update `test_slim_handler()` so that lastest pypi version is *not* hardcoded (get from "git tags" result)

* 🎨 run black/isort

* 🔧 attempt to update github action to retrieve the latest tags.

* 🔧 fix bad ci command definition

* 📝 created and added issue for python3.6 support removal.

* ✨ add `python_requires` to setup().

* 📝 fix issue zappa#900

* 📝 add link to https://slackautoinviter.herokuapp.com to join slack. (the zappateam.slack may be private?)

* 🔥 remove `future` (past) requirements basestring usage, replacing with isinstance(x, str)
🔧 change usage of utcnow() -> datetime.datetime.now(datetime.timezone.utc)

* 🔥 Remove 'futures' package requirement (zappa#826)

* 📝 fix issue zappa#900

* 🔥 remove unnecessary __future__ import
🔧 Start versioning at 0; 0.55.1 -> 0.55.0

* 🎨 fix flake8

* 🐛 fix cd.yaml to only run on push of tag meeting the release tag criteria. (zappa#1152)

* 📝 add 1152 to CHANGELOG

🔧 upgrade __version__ from 0.54.1 -> 0.54.2

Release 'Zappa' v0.54.1 (zappa#1070)

Co-authored-by: Alex DelVecchio <[email protected]>

v0.54.0

* Pin troposphere version and update to 3.x (zappa#1029)
* Relax stage name restrictions when not using apigateway (zappa#993)
* Wait for lambda to become active during deploy/update (zappa#992)
* add support for Python 3.9 (zappa#1026)

Merge pull request zappa#967 from ian-whitestone/add-docker-support

Update/deploy a lambda using a Docker container image