[INFRA] New attempt to silence LGTM false positive

Previous commit bb6065a from #853 does not work around the LGTM alert.
We attempt to silence the LGTM alert using an lgtm.yml file, and use
standard Flake8 noqa suppression comments to document the issue at hand.

.lgtm.yml

@@ -0,0 +1,4 @@
+---
+queries:
+  # https://lgtm.com/rules/6770079/
+  - exclude: py/unused-import

pdf_build_src/process_markdowns.py

@@ -16,7 +16,8 @@
 import numpy as np
 
 sys.path.append("../tools/")
-from mkdocs_macros_bids import macros  # noqa   (used in "eval" call later on)
+# functions from module macros are called by eval() later on
+from mkdocs_macros_bids import macros  # noqa: F401
 
 
 def run_shell_cmd(command):