TST: Old issues

[mroeschke]

• closes Pandas different between Python 2.7 and Python 3.5 #16380
• closes read_fwf does not use comment character if colspecs argument does not include first column #14135
• closes BUG: groupby.rank with non-unique index groupers #16577
• closes df.stack() still misbehaving in unsorted case #16925
• closes Ambiguous behaviour when transform groupby with NaNs #17093
• closes Aggregate mean over TimeDelta column #17382
• closes Comparison of categorical with tuple generates wrong results #18050
• tests added / passed
• Ensure all linting tests pass, see here for how to run them

[jbrockmendel]

CI failure looks unrelated

[jreback]

thanks @mroeschke