update _valid_key_re

[dongweiming]

ref: #118

Is it more appropriate to use this?

In [8]: _valid_key_re = re.compile(b'^[^\x00-\x20\x7f]{1,250}$')                                                  

In [9]: _valid_key_re.match(b'123')                                                                               
Out[9]: <_sre.SRE_Match object; span=(0, 3), match=b'123'>

In [10]: _valid_key_re.match(b'abc')                                                                               
Out[10]: <_sre.SRE_Match object; span=(0, 3), match=b'abc'>

In [11]: _valid_key_re.match(bytes('中文', 'utf-8'))                                                               
Out[11]: <_sre.SRE_Match object; span=(0, 6), match=b'\xe4\xb8\xad\xe6\x96\x87'>

In [12]: _valid_key_re.match(bytes('こんにちは', 'utf-8'))                                                         
Out[12]: <_sre.SRE_Match object; span=(0, 15), match=b'\xe3\x81\x93\xe3\x82\x93\xe3\x81\xab\xe3\x81\xa1>

In [13]: _valid_key_re.match(bytes('안녕하세요', 'utf-8'))                                                         
Out[13]: <_sre.SRE_Match object; span=(0, 15), match=b'\xec\x95\x88\xeb\x85\x95\xed\x95\x98\xec\x84\xb8>

In [14]: _valid_key_re.match(b' ')      

In [15]: _valid_key_re.match(bytes('!@#', 'utf-8'))                                                                
Out[15]: <_sre.SRE_Match object; span=(0, 3), match=b'!@#'>

[codecov]

Codecov Report

Merging #119 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #119   +/-   ##
======================================
  Coverage    91.5%   91.5%           
======================================
  Files           5       5           
  Lines         259     259           
  Branches       38      38           
======================================
  Hits          237     237           
  Misses         11      11           
  Partials       11      11

Impacted Files	Coverage Δ
aiomcache/client.py	88.29% <100%> (ø)	⬆️

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[dongweiming]

@fafhrd91 @asvetlov please review, thanks!

[rexzhang]

i have merged your PR to my project rexzhang@27c737d

[Dreamsorcerer]

Doc line needs updating.

[Dreamsorcerer]

As per the above comment and the memcached documention, "the key must not include
control characters or whitespace."

Therefore this will need some more work to allow non-ascii characters.

[Dreamsorcerer]

Fixed with #240 instead.