Pylint: false positives on member existence for internal classes

[reidpr]

Split from #1860. Pylint:

• E1101 no-member
• W0201 attribute-defined-outside-init

We get E1101 quite a lot, e.g.:

charliecloud/lib/build_cache.py

Lines 360 to 362 in d679373

	if ( stat.S_ISREG(fm.mode)
	or stat.S_ISLNK(fm.mode)
	or stat.S_ISFIFO(fm.mode)):

charliecloud/lib/build.py

Line 553 in d679373

return super().sid_input + self.src_metadata

In all these cases, the member does exist but it’s set in a strange way. This lint error is very powerful, so I want to make it useful. This issue is to eliminate the false positives so we can enable the message.

Similarly for W0201:

charliecloud/lib/build_cache.py

Lines 576 to 577 in d679373

	if (not (hasattr(self, "size"))):
	self.size = -1

This warning basically says that there is no list of attributes that can be consulted, expecting that in __init__(). For this particular class there is a nice comment explaining all the attributes, but some of them are also set to None in __init__(). Probably should fix all these warnings.