`planemo lint` wrongly warns about `strict` attribute of `<command>`

```
$ planemo lint ~/galaxy_blast/tools/ncbi_blast_plus/ncbi_makeprofiledb.xml
...
Applying linter command... WARNING
.. WARNING: Unknown attribute [strict] encountered on command tag.
```
The `strict` attribute is documented at https://docs.galaxyproject.org/en/latest/dev/schema.html#strict

May be of interest for @peterjc .