Allow x token to be used in formatting

[alstomli]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 📚 Updated documentation for changed code.
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Description of Changes

[codecov-io]

Codecov Report

Merging #737 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #737   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines        1678   1682    +4     
  Branches      283    284    +1     
=====================================
+ Hits         1678   1682    +4

Impacted Files	Coverage Δ
arrow/formatter.py	100% <100%> (ø)	⬆️
arrow/locales.py	100% <0%> (ø)	⬆️

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[systemcatch]

Hey @alstomli I've been checking out your branch and noticed that this is occurring.

(arrow) chris@ThinkPad:~/arrow$ python
Python 3.7.4 (default, Sep 19 2019, 11:01:37) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import arrow
>>> arw=arrow.utcnow()
>>> arw
<Arrow [2019-12-22T19:44:08.492814+00:00]>
>>> arw.format("x")
'x'

[alstomli]

I have just tested the program on my machine it seems there is nothing wrong
`>>> import arrow

arrow.utcnow().format("x")
'1577074797676671'
arw = arrow.utcnow()
arw.format("x")
'1577074859157219'
arw
<Arrow [2019-12-23T04:20:59.157219+00:00]>`

[alstomli]

Hi @systemcatch if we need to add new "x" format I think we need to rewrite
r"(\[(?:(?=(?P<literal>[^]]))(?P=literal))*\]|YYY?Y?|MM?M?M?|Do|DD?D?D?|d?dd?d?|HH?|hh?|mm?|ss?|SS?S?S?S?S?|ZZ?Z?|a|A|X)" into r"(\[(?:(?=(?P<literal>[^]]))(?P=literal))*\]|YYY?Y?|MM?M?M?|Do|DD?D?D?|d?dd?d?|HH?|hh?|mm?|ss?|SS?S?S?S?S?|ZZ?Z?|a|A|X|x)" so the program can detect the new token, however, when I submitted this change it says it is an conflit, is there any way to solve this?

[jadchaar]

Hey, we are going to close this for now since we are pushing these changes to when we drop Python 2.7 support because this will allow us to introduce some breaking changes to the existing API.