[setup] add fallback version

We are trying to lock down the `python-requirements.txt` file in the
lowRISC/opentitan repository by adding hashes for all dependencies
(including transitive dependencies), to satify OpenTitan partner
organization-level requirements. To do so, we would like to generate a
python-requirements.txt file with the required hashes using the
`pip-compile` command from the `pip-tools` package.

The way the version is detected in the `setup.py` configuration in this
package is using the `use_scm_version` feature which attempts to extract
the version from VCS (i.e., git) metadata. However, this requires installing
the package with `pip install git+https://github.com/lowRISC/fusesoc.git@<tag>`.
However, to use the `pip-compile` command to generate secure
`python-requirements.txt` file (i.e., one with pinned hashes), requires
installing packages directly using the HTTPS URL syntax, i.e.,
`pip install https://github.com/lowRISC/fusesoc/archive/refs/tags/ot-0.3.zip`,
i.e., bypassing git. Unfortunately, this fails to find the version,
since there is no git metadata to parse.

Therefore, I updated the `setup.py` configuration to use a
"fallback_version".

Signed-off-by: Tim Trippel <ttrippel@google.com>

setup.py

@@ -17,6 +17,7 @@ def read(fname):
     use_scm_version={
         "relative_to": __file__,
         "write_to": "fusesoc/version.py",
+        "fallback_version": "0.4",
     },
     author="Olof Kindgren",
     author_email="olof.kindgren@gmail.com",