add import expression support

[ioistired]

If I can figure out how to make a damn link in rST I will document this too.

[ioistired]

For more information on Import Expression Parser please see https://github.com/bmintz/import-expression-parser.

Take note of these known issues:

1. The rejection of multiple ! in one dotted attribute access is inconsistent at this time:

• a.b.c!.d! is not rejected
• a.b!.c!.d! is rejected
• a!.b!.c!.d is rejected
• a!.b!.c.d is not rejected

These cases act as though only the last ! is present, e.g. a!.b!.c.d is a.b!.c.d.
2. Some other invalid syntax, such as import x! and def foo(*, x!) is not rejected yet.
3. Import expressions within f-strings, such as f'Message creation: {discord.utils!.snowflake_time(ctx.message.created_at)}' are not supported at all. This is due to f-strings being hell to parse, and ! already being supported inside certain f-string format specifiers.

Issues (1) and (2) can be disregarded if you anticipate the user not doing these things, and indeed import collections! will still fail, but with an unexpected error: ModuleNotFoundError: No module named 'collections__IMPORT_EXPR_END'.

Sadly def foo(*, x!) will currently not fail until the user tries to use the function:

>>> import_expression.exec('def foo(*, x!): pass')
>>> inspect.signature(foo)
<Signature (*, x__IMPORT_EXPR_END)>
>>> foo(x=1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() got an unexpected keyword argument 'x'

I consider this to be a less acceptable failure since it works until later on. If this is a deal breaker for you, please consider leaving the PR open until I fix this issue, as the issue would be fixed on pypi and not in this PR.