Glean-parser

- 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.

- **Breaking change:** glinter errors found during the `translate` command will
now return an error code. glinter warnings will be displayed, but not return
an error code.
- `glean_parser` now produces a linter warning when `user` lifetime metrics are
set to expire. See [bug
1604854](https://bugzilla.mozilla.org/show_bug.cgi?id=1604854) for additional
context.

- **Breaking change:** The regular expression used to validate labels is
stricter and more correct.
- Add more information about pings to markdown documentation:
- State whether the ping includes client id;
- Add list of data review links;
- Add list of related bugs links.
- `glean_parser` now makes it easier to write external translation
functions for different language targets.
- BUGFIX: `glean_parser` now works on 32-bit Windows.

- Dropped the `inflection` dependency.
- Constrained the `zipp` and `MarkupSafe` transitive dependencies to versions
that support Python 3.5.