custom_checker doesn't work for pylint

[yegor256]

After #224. I run make clean lint and I see this:

+++ lint +++

Citation metadata are valid according to schema version 1.2.0.
mypy.ini: No [mypy] section in config file
Success: no issues found in 6 source files
************* Module Command line or configuration file
Command line or configuration file:1:0: E0013: Plugin 'custom_checkers' is impossible to load, is it installed ? ('No module named 'custom_checkers'') (bad-plugin-value)

Apparently, pylint doesn't load custom_checkers.

[yegor256]

@howcanunot can you please help?

[howcanunot]

@yegor256 Yeah, I'm sorry.
fix is ready

[yegor256]

@howcanunot works now, thanks