chore: py2 compatibility code cleanup

[ankush]

removed dependency and all code related to following packages. Replaced compatibility wrapper calls with py3 style e.g. iteritems(dict) => dict.items()

• future
• six

[codecov]

Codecov Report

Merging #28249 (37e6e1f) into develop (25f647f) will increase coverage by 11.02%.
The diff coverage is 43.47%.

@@             Coverage Diff              @@
##           develop   #28249       +/-   ##
============================================
+ Coverage    44.17%   55.20%   +11.02%     
============================================
  Files         1248     1117      -131     
  Lines        67757    66491     -1266     
============================================
+ Hits         29931    36704     +6773     
+ Misses       37826    29787     -8039     

Impacted Files	Coverage Δ
erpnext/__init__.py	69.87% <ø> (+2.02%)	⬆️
...count_balance_timeline/account_balance_timeline.py	0.00% <ø> (ø)
erpnext/accounts/deferred_revenue.py	76.00% <ø> (+18.47%)	⬆️
erpnext/accounts/doctype/account/account.py	70.98% <ø> (+33.08%)	⬆️
...e/account/chart_of_accounts/import_from_openerp.py	0.00% <0.00%> (ø)
...of_accounts/verified/standard_chart_of_accounts.py	100.00% <ø> (ø)
.../standard_chart_of_accounts_with_account_number.py	100.00% <ø> (ø)
...ctype/accounting_dimension/accounting_dimension.py	65.62% <ø> (+34.61%)	⬆️
...ng_dimension_detail/accounting_dimension_detail.py	100.00% <ø> (+100.00%)	⬆️
...ng_dimension_filter/accounting_dimension_filter.py	95.45% <ø> (+38.93%)	⬆️
... and 1235 more