Fix Fuzzing Build Failure Due to Outdated pip

The pre-installed version of `pip` (19.x) was outdated and unable to
parse the `pyproject.toml` syntax during the install step in `build.sh`
causing the script to error out and crash.

Upgrading `pip` to the latest version in the project image resolves the
issue and allows the installation to complete.

[1]: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=67399

projects/gitpython/Dockerfile

@@ -13,6 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 FROM gcr.io/oss-fuzz-base/base-builder-python
-RUN git clone https://github.com/gitpython-developers/gitpython gitpython
+RUN git clone https://github.com/gitpython-developers/gitpython gitpython \
+    && python3 -m pip install --upgrade pip;
+
 COPY *.sh *py $SRC/
 WORKDIR $SRC/gitpython

projects/gitpython/build.sh

@@ -14,7 +14,7 @@
 # limitations under the License.
 #
 ################################################################################
-pip3 install .
+python3 -m pip install .
 
 # Build fuzzers in $OUT.
 for fuzzer in $(find $SRC -name 'fuzz_*.py'); do