CLN/DEPR: Fix instances of 'U'/'rU' in open(...)

#7131
BUG: xlim on plots with shared axes (GH2960, GH3490)
REGR: Add back #N/A N/A as a default NA value (regresion from 0.12) (GH5521)
Merge pull request #7639 from jreback/na_values

REGR: Add back #N/A N/A as a default NA value (regresion from 0.12) (GH5521)
Merge pull request #7625 from bwignall/depr_ru

CLN/DEPR: Fix instances of 'U'/'rU' in open(...)
Merge pull request #7322 from rosnfeld/xlim_issues

BUG: xlim on plots with shared axes (GH2960, GH3490)
BUG: Error in rolling_var if window is larger than array, fixes #7297
Merge pull request #7572 from jaimefrio/rolling_var_bug

BUG: Error in rolling_var if window is larger than array, fixes #7297
TST: fixes for 2.6 comparisons
Revert "Merge pull request #7591 from mcwitt/parse-index-cols-c"

This reverts commit 2c603e17b60665b5db6b373f33e1479f47b7155a, reversing
changes made to 49a86f1fb90ea199cba0954ede6b939f672867b1.
PERF: allow dst transition computations to be handled much faster
      if the end-points are ok (GH7633)
PERF: allow slice indexers to be computed faster
Merge pull request #7652 from jreback/dst_transitions

PERF: fix perf issue in tz conversions w/o affecting DST transitions