Glean-parser

- **Breaking change:** (Swift only) Combine all metrics and pings into a single
generated file `Metrics.swift`.

- `glinter` messages have been improved with more details and to be more
actionable.
- A maximum of 10 `extra_keys` is now enforced for `event` metric types.
- BUGFIX: the `Lifetime` enum values now match the values of the implementation
in mozilla/glean.

- BUGFIX: yamllint errors are now reported using the correct file name.

- Support for using `timing_distribution`'s `time_unit` parameter to control
the range of acceptable values is documented. The default unit for this use
case is `nanosecond` to avoid creating a breaking change. See [bug
1630997](https://bugzilla.mozilla.org/show_bug.cgi?id=1630997) for more
information.

- Dependencies that depend on the version of Python being used are now specified
using the [Declaring platform specific dependencies syntax in
setuptools](https://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-platform-specific-dependencies).
This means that more recent versions of dependencies are likely to be
installed on Python 3.6 and later, and unnecessary backport libraries won't
be installed on more recent Python versions.

- The minimum version of the runtime dependencies has been lowered to increase
compatibility with other tools. These minimum versions are now tested in CI,
in addition to testing the latest versions of the dependencies that was
already happening in CI.