hghave: adjust the detection of `pylint` for modern versions

I have pylint 2.6.0 installed locally, and it only has a single space.  I have
no idea when this changed.

Differential Revision: https://phab.mercurial-scm.org/D9431

tests/hghave.py

@@ -586,7 +586,7 b' def has_pyflakes():'
 
 @check("pylint", "Pylint python linter")
 def has_pylint():
-    return matchoutput("pylint --help", br"Usage:  pylint", True)
+    return matchoutput("pylint --help", br"Usage:[ ]+pylint", True)
 
 
 @check("clang-format", "clang-format C code formatter")