tqdm: fix casting to boolean to mimic self.iterable

[immerrr]

I have hit an issue described in #353 today with what can be boiled down to:

In [28]: bool(tqdm.tqdm((1 for i in range(10))))                                                                                                                                              
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-28-e62055aa18f4> in <module>
----> 1 bool(tqdm.tqdm((1 for i in range(10))))

TypeError: 'NoneType' object cannot be interpreted as an integer

In [29]:   import tqdm, sys 
    ...:   print(tqdm.__version__, sys.version, sys.platform)                                                                                                                                 
4.29.1 3.6.6 (default, Oct  9 2018, 14:19:21) 
[GCC 5.4.0 20160609] linux

This is a stab at fixing it: it will make the tqdm wrapper behave in exactly the same way as the wrapped self.iterable when cast to boolean. Admittedly, it could be made more intelligent by comparing the current position to the total length if it is known, but that would complicate things and I wanted to start with a simple solution.

• I have visited the source website, and in particular
  read the known issues
• I have searched through the issue tracker for duplicates
• I have mentioned version numbers, operating system and
  environment, where applicable:

  import tqdm, sys
  print(tqdm.__version__, sys.version, sys.platform)

• If applicable, I have mentioned the relevant/related issue(s)

[immerrr]

I have reviewed the failures and realized that I somehow missed the test_bool function that already tests the boolean cast behaviour that prioritizes self.total over the boolean cast for self.iterable, so I have pushed a change to __bool__ operator that does that.

The existing test also tests for TypeError if the tqdm has no self.iterable, but doesn't test it for sizeless iterators, so I have adjusted my PR to play nice with the existing expectations.

[codecov-io]

Codecov Report

Merging #694 into devel will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##            devel    #694      +/-   ##
=========================================
+ Coverage   98.58%   98.6%   +0.01%     
=========================================
  Files          10      10              
  Lines         779     787       +8     
  Branches      141     143       +2     
=========================================
+ Hits          768     776       +8     
  Misses          6       6              
  Partials        5       5

[casperdcl]

thanks.