Pin idna to latest version 2.7

[fstab50]

This PR pins idna to the latest release 2.7.

Changelog

Changelog

2.7

- Update to Unicode 10.0.0.
- No longer accepts dot-prefixed domains (e.g. ".example") as valid.
This is to be more conformant with the UTS 46 spec. Users should
strip dot prefixes from domains before processing.```



### 2.6
```++++++++++++++++

- Allows generation of IDNA and UTS 46 table data for different
versions of Unicode, by deriving properties directly from
Unicode data.
- Ability to generate RFC 5892/IANA-style table data
- Diagnostic output of IDNA-related Unicode properties and
derived calculations for a given codepoint
- Support for idna.__version__ to report version
- Support for idna.idnadata.__version__ and
idna.uts46data.__version__ to report Unicode version of
underlying IDNA and UTS 46 data respectively.```



### 2.5
```++++++++++++++++

- Fix bug with Katakana middle dot context-rule (Thanks, Greg
Shikhman.)```



### 2.4
```++++++++++++++++

- Restore IDNAError to be a subclass of UnicodeError, as some users of
this library are only looking for the latter to catch invalid strings.```



### 2.3
```++++++++++++++++

- Fix bugs relating to deriving IDNAError from UnicodeError.
- More memory footprint improvements (Thanks, Alex Gaynor)```



### 2.2
```++++++++++++++++

- Made some changes to the UTS 46 data that should allow Jython to get around
64kb Java class limits. (Thanks, John A. Booth and Marcin Płonka.)
- In Python 2.6, skip two tests that rely on data not present in that
Python version's unicodedata module.
- Use relative imports to help downstream users.```



### 2.1
```++++++++++++++++

- Memory consumption optimizations. The library should consume significantly
less memory through smarter data structures being used to represent
relevant Unicode properties. Many thanks to Shivaram Lingamneni for this
patch.
- Patches to make library work better with Python 2.6. The core library
currently works however the unit testing does not. (Thanks, Robert
Buchholz)
- Better affix all Unicode codepoint properties to a specific version.```



### 2.0
```++++++++++++++++

- Added support for Unicode IDNA Compatibility Processing (aka Unicode
Technical Standard 46). Big thanks to Jon Ribbens who contributed this
functionality.```



### 1.1
```++++++++++++++++

- Use IDNA properties from Unicode 6.3.0. Internet Architecture Board (IAB)
issued statement recommending against the use of Unicode 7.0.0 until
issues relating to U+08A1 codepoint are resolved. See http://goo.gl/Ed1n0K
- Identify some cases when label would be too longer to be a legal DNS name
and raise an exception. (Thanks, Ed Lewis)```



### 1.0
```++++++++++++++++

- Update IDNA properties for Unicode 7.0.0.```



### 0.9
```++++++++++++++++

- Fix issue with non-UTF-8 environments reading the README file
now that it contains non-ASCII. (Thanks, Tom Prince)
- Codec functions are useful, so they are separated into their own
module, rather than just existing for compatibility reasons.
- Add LICENSE file.```



### 0.8
```++++++++++++++++

- Added MANIFEST.in for correct source distribution compilation.```



### 0.7
```++++++++++++++++

- Filled out missing tests for various functions.
- Fix bug in CONTEXTO validation for Greek lower numeral sign (U+0375)
- Fix bug in CONTEXTO validation for Japanese middle dot (U+30FB)
- Improved documentation
- Move designation to Stable```



### 0.6
```++++++++++++++++

- Minor improvements to Python 3 support, tests (Thanks, Derek Wilson)```



### 0.5
```++++++++++++++++

- Update IDNA properties for Unicode 6.3.0.```



### 0.4
```++++++++++++++++

- Fix trove classifier for Python 3. (Thanks, Hynek Schlawack)```



### 0.3
```++++++++++++++++

- Ported to Python 3.```



### 0.2
```++++++++++++++++

- Improve packaging.
- More conformant, passes all relevant tests in the Unicode TR46 test suite.```



### 0.1
```++++++++++++++++

- First proof-of-concept version.```


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