Manheim-c7n-tools

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* Merge `PR 34 <https://github.com/manheim/manheim-c7n-tools/pull/34>`__ to add an optional full HTML report to the ``dryrun-diff`` step/entrypoint, triggered by the presence of a ``reporting-template/report.j2`` template file. Many thanks to `JuubiSnake <https://github.com/JuubiSnake>`__ of `FundingCircle <https://github.com/FundingCircle>`__ for the contribution.
* In recognition of this project being used extensively in our organization, bump version to 1.0.0.

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* Bump c7n and c7n-mailer installed version to `b62af99 <https://github.com/cloud-custodian/cloud-custodian/commit/b62af99171bf1163413d7f7411e4a0db8a50f27e>`__ in order to pull in `PR #5751 <https://github.com/cloud-custodian/cloud-custodian/pull/5751>`__, fix for `#5750 <https://github.com/cloud-custodian/cloud-custodian/issues/5750>`__ c7n-mailer critical bug.
* Add unit test to ensure that the ``c7n-mailer`` Lambda package archive can be successfully generated.
* Add ``pip freeze`` to the end of Docker image tests, to keep a record of the packages installed in Docker in the build logs.

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* Install c7n and c7n-mailer directly from github, instead of PyPI, to pull in unreleased-but-merged `b7178be <https://github.com/cloud-custodian/cloud-custodian/commit/b7178be718bd8c8bdb70b2376d3bb0d5eb6fa9a9>`__ / `PR #5708 <https://github.com/cloud-custodian/cloud-custodian/pull/5708>`__ which fixes `Issue #5707 <https://github.com/cloud-custodian/cloud-custodian/issues/5707>`__ for missing ``jsonpointer`` and ``jsonpatch`` dependencies.
* Remove ``jsonpointer`` from requirements.
* Add ``libffi-dev`` and ``openssl-dev`` build dependencies to Dockerfile.
* Add ``.dockerignore`` file to make Docker builds more efficient.

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* Add ``jsonpointer`` to requirements.

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**Important:** In following upstream c7n's `0.9.1.0 release <https://github.com/cloud-custodian/cloud-custodian/releases/tag/0.9.1.0>`__, this release drops support for Python 2.7. A modern version of Python 3 is now required.

* Upgrade `c7n-mailer <https://github.com/cloud-custodian/cloud-custodian/tree/master/tools/c7n_mailer>`__ requirement from 0.5.7 to 0.6.0.
* Upgrade ``c7n`` requirement from 0.8.46.0 to 0.9.1.0.
* Switch TravisCI tests from py36 and py37 to py37 and py38.
* Update vendored-in ``mugc`` with latest upstream version, for compatibility with above changes.