Allow importing exceptions from nested modules

[althonos]

Some exceptions are not available in a top-level Python namespace (for instance exceptions defined in email.errors) but the current implementation of the import_exception macro didn't allow for dotted module names.

I added a macro to stringify a dotted path (dot_stringify!(a.b.c) => "a.b.c") and used to fix the import_exception macro. It's backward compatible with the current implementation.

As a test, I left the current test but added a test with MessageError exception from the email.errors module (which is available in the standard library of all Python version this library supports).

[althonos]

I should probably add a little extra to the PyO3 guide as well.

[fafhrd91]

looks good, but could you rebase against master

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@d50d1fb). Click here to learn what that means.
The diff coverage is 96.15%.

@@            Coverage Diff            @@
##             master     #116   +/-   ##
=========================================
  Coverage          ?   80.91%           
=========================================
  Files             ?       81           
  Lines             ?     4961           
  Branches          ?        0           
=========================================
  Hits              ?     4014           
  Misses            ?      947           
  Partials          ?        0

Impacted Files	Coverage Δ
src/typeob.rs	86.12% <ø> (ø)
src/lib.rs	100% <ø> (ø)
src/ffi3/objimpl.rs	0% <0%> (ø)
src/objectprotocol.rs	74.43% <100%> (ø)
src/objects/exc_impl.rs	100% <100%> (ø)

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