chore: Adding header-checker-lint and updating environment (#73)

* changes without context

        autosynth cannot find the source of changes triggered by earlier changes in this
        repository, or by version upgrades to tools such as linters.

* chore(python): skip docfx in main presubmit

* chore(python): skip docfx in main presubmit

* fix: properly template the repo name

Source-Author: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
Source-Date: Fri Jan 8 10:32:13 2021 -0700
Source-Repo: googleapis/synthtool
Source-Sha: fb53b6fb373b7c3edf4e55f3e8036bc6d73fa483
Source-Link: googleapis/synthtool@fb53b6f

* chore: add missing quotation mark

Source-Author: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
Source-Date: Mon Jan 11 09:43:06 2021 -0700
Source-Repo: googleapis/synthtool
Source-Sha: 16ec872dd898d7de6e1822badfac32484b5d9031
Source-Link: googleapis/synthtool@16ec872

* chore: add 3.9 to noxfile template

Since the python-docs-samples noxfile-template doesn't sync with this, I wanted to make sure the noxfile template matched the most recent change [here](https://github.com/GoogleCloudPlatform/python-docs-samples/pull/4968/files)

cc @tmatsuo

Source-Author: Leah E. Cole <6719667+leahecole@users.noreply.github.com>
Source-Date: Fri Jan 15 17:24:05 2021 -0800
Source-Repo: googleapis/synthtool
Source-Sha: 56ddc68f36b32341e9f22c2c59b4ce6aa3ba635f
Source-Link: googleapis/synthtool@56ddc68

* build(python): make `NOX_SESSION` optional

I added this accidentally in #889. `NOX_SESSION` should be passed down if it is set but not marked required.

Source-Author: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
Source-Date: Tue Jan 19 09:38:04 2021 -0700
Source-Repo: googleapis/synthtool
Source-Sha: ba960d730416fe05c50547e975ce79fcee52c671
Source-Link: googleapis/synthtool@ba960d7

* chore: Add header checker config to python library synth

Now that we have it working in [python-docs-samples](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/header-checker-lint.yml) we should consider adding it to the 🐍 libraries :)

Source-Author: Leah E. Cole <6719667+leahecole@users.noreply.github.com>
Source-Date: Mon Jan 25 13:24:08 2021 -0800
Source-Repo: googleapis/synthtool
Source-Sha: 573f7655311b553a937f9123bee17bf78497db95
Source-Link: googleapis/synthtool@573f765

* chore: add noxfile parameters for extra dependencies

Also, add tests for some noxfile parameters for assurance that the
template generates valid Python.

Co-authored-by: Jeffrey Rennie <rennie@google.com>

Source-Author: Tim Swast <swast@google.com>
Source-Date: Tue Jan 26 12:26:57 2021 -0600
Source-Repo: googleapis/synthtool
Source-Sha: 778d8beae28d6d87eb01fdc839a4b4d966ed2ebe
Source-Link: googleapis/synthtool@778d8be

error_reporting/fluent_on_compute/noxfile.py

@@ -85,7 +85,7 @@ def get_pytest_env_vars() -> Dict[str, str]:
 
 # DO NOT EDIT - automatically generated.
 # All versions used to tested samples.
-ALL_VERSIONS = ["2.7", "3.6", "3.7", "3.8"]
+ALL_VERSIONS = ["2.7", "3.6", "3.7", "3.8", "3.9"]
 
 # Any default versions that should be ignored.
 IGNORED_VERSIONS = TEST_CONFIG['ignored_versions']