Update agate requirement from <1.6.2,>=1.6 to >=1.6,<1.6.4 in /core

Updates the requirements on [agate](https://github.com/wireservice/agate) to permit the latest version.
- [Release notes](https://github.com/wireservice/agate/releases)
- [Changelog](https://github.com/wireservice/agate/blob/master/CHANGELOG.rst)
- [Commits](wireservice/agate@1.6.0...1.6.3)

---
updated-dependencies:
- dependency-name: agate
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

core/setup.py

@@ -51,7 +51,7 @@ def read(fname):
     install_requires=[
         'Jinja2==2.11.3',
         'PyYAML>=3.11',
-        'agate>=1.6,<1.6.2',
+        'agate>=1.6,<1.6.4',
         'colorama>=0.3.9,<0.4.5',
         'dataclasses>=0.6,<0.9;python_version<"3.7"',
         'hologram==0.0.14',