Fix calendars extra

[noviluni]

I noticed that pip install dateparser[calendars] didn't work. After some time investigating, it seems that there was a typo in extras_require.

I also updated the tox file after this change and the docs (that were wrong). The requirements.txt file will be deleted in this PR: #739

[codecov]

Codecov Report

Merging #740 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #740   +/-   ##
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  Coverage   95.34%   95.34%           
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  Files         304      304           
  Lines        2622     2622           
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  Hits         2500     2500           
  Misses        122      122           

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[noviluni]

I added a temporary workaround (pinning a working regex package version) to make the Pypy pipeline pass.

As mentioned here: #565, it seems that dateparser doesn't work with Pypy out of the box because of the regex library which we depend on.

The next version will remove Python 2.7 support: #727 (and Pypy), so I hope we will remove this requirement pinning before releasing the next version.

I will investigate if this affects pypy3 to know if we can support it or not.