Don't pass '--upgrade' to pip when scanning a requirements file. (#40)

It's no longer required, and can cause problems with some packages (like 'xgboost' on Windows).

spotfire/spk.py

@@ -323,7 +323,7 @@ def scan_requirements_txt(self, requirements: str, constraint: str, prefix: str,
 
         # Install the packages from the requirement file into tempdir.
         _message(f"Installing pip packages from {requirements} to temporary location.")
-        command = [sys.executable, "-m", "pip", "install", "--upgrade", "--disable-pip-version-check",
+        command = [sys.executable, "-m", "pip", "install", "--disable-pip-version-check",
                    "--target", tempdir, "--requirement", requirements]
         if constraint:
             command.extend(["--constraint", constraint])