Catch exceptions in _check_module

[vstinner]

• _check_module.main() now catchs IndexError and FileNotFoundError
  errors: raise ArgcompleteMarkerNotFound exception instead.
• Replace open() with tokenize.open() to handle "# coding: xxx"
  encoding cookie: support source code using an encoding different
  than UTF-8 on Python 3.

[vstinner]

On Fedora, a default exception hook is installed on the whole system. Uncatched exceptions are reported to Fedora using ABRT. Type "python3 -m http.ser[TAB]" on Fedora detects a bug and invites the user to report the bug to Fedora. Example of such report:
https://bugzilla.redhat.com/show_bug.cgi?id=1737212

Bonus: this change also adds support for encoding different than UTF-8 on Python 3. Tell me if you prefer a separated PR for that.

Note: I tested my change on Python 3.7 using my other PR #268.

[codecov-io]

Codecov Report

Merging #269 into master will decrease coverage by 1.37%.
The diff coverage is 6.66%.

@@            Coverage Diff             @@
##           master     #269      +/-   ##
==========================================
- Coverage   83.85%   82.47%   -1.38%     
==========================================
  Files           7        7              
  Lines         706      719      +13     
==========================================
+ Hits          592      593       +1     
- Misses        114      126      +12

Impacted Files	Coverage Δ
argcomplete/_check_module.py	58.73% <6.66%> (-13.27%)	⬇️

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[vstinner]

I rebased this PR on top on my tox change.

Oops, argcomplete supports Python 2: I didn't notce that. I adapted my PR for Python 2.

[evanunderscore]

Thanks for the PR @vstinner! I'm happy to merge this for you, however I won't be able to push a release.

[vstinner]

I forgot this PR that I wrote 2 months ago. Feel free to modify it if you want :-)

[kislyuk]

@evanunderscore your discretion with this PR, please merge as you see fit and I will cut a release when ready.

[evanunderscore]

Thanks for your work @vstinner!