[lit] Add pyproject.toml and fix build w/ modern setuptools backend

Add a `pyproject.toml` file that provides build system information
for PEP 517-compliant builders.  While all the commonly used builders
provide fallback to running `setup.py` for backwards compatibility, this
ensures the best forward compatibility.  It also provides a reliable way
of specifying the minimum required setuptools version.  Effectively, it
will make it possible to remove `setup.py` in favor of purely
declarative configuration in the future, or even switch to a different
build system.

Update `setup.py` to explicitly add the current directory to `sys.path`
for importing `lit`.  This is necessary, as the modern setuptools
backend does not guarantee that the current directory is present there.

Differential Revision: https://reviews.llvm.org/D136976

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