Create .gitattributes

[cedric-anne]

Add .gitattributes to exclude files/directories from dist package.

[codecov]

Codecov Report

Merging #191 (e91aba4) into master (9976ac3) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #191   +/-   ##
=========================================
  Coverage     94.11%   94.11%           
  Complexity      260      260           
=========================================
  Files            15       15           
  Lines           884      884           
=========================================
  Hits            832      832           
  Misses           52       52           

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[phil-davis]

Note: this means that the test code is not in the release distribution package, which is nicer for production installs (they don't want or need all that extra crud)

Some upstream CI tries to run the sabre/http unit tests. For example sabre/dav sabre-io/dav#1439 can't find the sabre/http unit tests.

That is all fine, and I will sort out what to do in sabre/dav. I am just making this comment here so that it might help future people who are searching for where this changed.