Glean-parser

- `glean_parser` now supports Python versions 3.5, 3.6, 3.7 and 3.8.

- The `translate` command will no longer clear extra files in the output
directory.
- BUGFIX: Ensure all newlines in comments are prefixed with comment markers
- BUGFIX: Escape Swift keywords in variable names in generated code
- Generate documentation for pings that are sent if empty

- Reserve the `deletion_request` ping name
- Added a new flag `send_if_empty` for pings

- The `glinter` command now performs `yamllint` validation on registry files.

- The Kotlin linter `detekt` is now run during CI, and for local
testing if installed.
- Python 3.8 is now tested in CI (in addition to Python 3.7). Using
`tox` for this doesn't work in modern versions of CircleCI, so the
`tox` configuration has been removed.
- `yamllint` has been added to test the YAML files on CI.
- ⚠ Metric types that don't yet have implementations in glean-core
have been removed. This includes `enumeration`, `rate`, `usage`, and
`use_counter`, as well as many labeled metrics that don't exist.

- Allow a Swift lint for generated code
- New lint: Restrict what metric can go into the `baseline` ping
- New lint: Warn for slight misspellings in ping names
- BUGFIX: change Labeled types labels from lists to sets.