Use thread pool for faster network IO

[pavdmyt]

Fetching package version info from PyPI can be much more faster by utilizing multiprocessing.dummy. Basically it's a wrapper around threading module which replicates multiprocessing API. It provides a convenient means to concurrently instantiate pypiup.requirement.Requirement objects which in turn make GET requests to PyPI. In short all code required for concurrent execution is only 4 lines:

# Setup pool of workers, qty of workers defaults to the number of CPU cores
pool = ThreadPool()
# Instantiate Requirement objects (and make GET requests) in their own threads
requirements = pool.map(Requirement, args)
# Close pool and wait all work to finish
pool.close()
pool.join()

Another part is to make progressbar work correctly in a concurrent mode. I've rewrote progressbar handling code a bit to manually update it's state. Basically click.progressbar setup step requires length param instead of iterable and update method should be called on bar each time we want to actually update it. To make sure that bar.update is called only after Requirement object instantiated and decouple this logic from Requirement class, I've implemented update functionality using barupdate decorator which automatically calls bar.update each time new Requirement object created.

Performance results

Below some results obtained by calculating time required for pypiup.requirements.Requirements.read_file execution:

Before:
-------

$ pypiup -r requirements/requirements-demo.txt  # (Total Requirements: 8)
read_file: 6.328423023223877

$ pypiup -r requirements-huge.txt  # (Total Requirements: 45)
read_file: 14.644872188568115

After:
------

$ pypiup -r requirements/requirements-demo.txt  # (Total Requirements: 8)
read_file: 0.9168651103973389

$ pypiup -r requirements-huge.txt  # (Total Requirements: 45)
read_file: 1.940899133682251

[codecov-io]

Codecov Report

Merging #14 into master will increase coverage by 0.06%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   99.18%   99.24%   +0.06%     
==========================================
  Files           4        4              
  Lines         123      133      +10     
==========================================
+ Hits          122      132      +10     
  Misses          1        1

Impacted Files	Coverage Δ
pypiup/requirements.py	100% <100%> (ø)	:arrow_up:

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[pavdmyt]

@manosim Any chance we can get this merged, or a similar PR?

[pavdmyt]

@manosim how about merging this PR as well? Sorry if pushing you, I'd like to have this in order to update my project's dev dependencies with newest pypiup ;)

[pavdmyt]

@manosim the PR is up to date with master now. Can you merge it please?