docs.rs failed to build the documentation of the recently released
rustpython-parser 0.2.0 because the build.rs script couldn't write the
parser.rs file because docs.rs builds the documentation in a sandbox
with a read-only filesystem.
This commit fixes this by writing the parser.rs file to the cargo output
directory instead, as recommended by the docs.rs documentation.[1]
Fixes#4436.
[1]: https://docs.rs/about/builds#read-only-directories
