Fix Python version inspection for pre-release (#706)

Author: uranusjr

docs/changelog.md

@@ -1,6 +1,7 @@
 dev
 
 - Fix to `pipx ensurepath` to fix behavior in user locales other than UTF-8, to fix #644. The internal change is to use userpath v1.6.0 or greater. (#700)
+- Fix virtual environment inspection for Python releases that uses an int for its release serial number. (#706)
 
 0.16.3
 

src/pipx/venv_inspect.py

@@ -172,7 +172,11 @@ def fetch_info_in_venv(venv_python_path: Path) -> Tuple[List[str], Dict[str, str
         impl_ver = sys.implementation.version
         implementation_version = "{0.major}.{0.minor}.{0.micro}".format(impl_ver)
         if impl_ver.releaselevel != "final":
-            implementation_version += impl_ver.releaselevel[0] + impl_ver.serial
+            implementation_version = "{}{}{}".format(
+                implementation_version,
+                impl_ver.releaselevel[0],
+                impl_ver.serial,
+            )
 
         sys_path = sys.path
         try: