Drop 'six' module

Drop 'u"' prefixes for text

Remove other Python 2.7 workarounds

Drop use of 'pytz'

Dxpand range to allow 'google-auth' 2.x versions

Remove 'general_helpers.wraps': except for a backward-compatibility
import, 'functools.wraps' does everything wee need on Python >= 3.6.

Remove 'packaging' dependency

Release-As: 2.0.0b1

Closes #74.

Closes #215.

See https://github.com/googleapis/repo-automation-bots/blob/63c858e539e1f4d9bb8ea66e12f9c0a0de5fef55/packages/sync-repo-settings/src/required-checks.json#L40-L50 for defaults.

Source-Link: googleapis/synthtool@7e1f6da
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:a1a891041baa4ffbe1a809ac1b8b9b4a71887293c9101c88e8e255943c5aec2d

Rely on the pins in 'setup.py' as the Source of Truth.

See #234 (review)

Source-Link: googleapis/synthtool@facee4c
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:9743664022bd63a8084be67f144898314c7ca12f0a03e422ac17c733c129d803

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Fixes #185 🦕

🤖 I have created a release \*beep\* \*boop\*
---
## [2.0.0](https://www.github.com/googleapis/python-api-core/compare/v2.0.0-b1...v2.0.0) (2021-08-18)


### ⚠ BREAKING CHANGES

* drop support for Python 2.7 / 3.5 ([#212](https://www.github.com/googleapis/python-api-core/issues/212)) ([a30f004](https://www.github.com/googleapis/python-api-core/commit/a30f004e74f709d46e905dd819c71f43354e9ac9))

### Bug Fixes

* bump grpcio version to use stable aio API ([#234](https://www.github.com/googleapis/python-api-core/issues/234)) ([bdbf889](https://www.github.com/googleapis/python-api-core/commit/bdbf889210b709d7c1945f2160bcba9161b4dd2e))
* strip trailing _ from field mask paths ([#228](https://www.github.com/googleapis/python-api-core/issues/228)) ([ff6ef1b](https://www.github.com/googleapis/python-api-core/commit/ff6ef1bd07fa68307b7c82c910416d770e7b3416))
---


This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please).

Closes #260

Co-authored-by: Tres Seaver <tseaver@palladion.com>

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>