Actually filter out missing interpreters (pypa#5262)

* Actually filter out missing interpreters

This fixes pypa#5261.

Before this change, I would get a "The Python interpreter can't be
found" error when running `pipenv install --system` with a python3 but
no python.

.github/PULL_REQUEST_TEMPLATE.md

@@ -19,7 +19,7 @@ How does this pull request fix your problem? Did you consider any alternatives?
 ### The checklist
 
 * [ ] Associated issue
-* [ ] A news fragment in the `news/` directory to describe this fix with the extension `.bugfix`, `.feature`, `.behavior`, `.doc`. `.vendor`. or `.trivial` (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.
+* [ ] A news fragment in the `news/` directory to describe this fix with the extension `.bugfix.rst`, `.feature.rst`, `.behavior.rst`, `.doc.rst`. `.vendor.rst`. or `.trivial.rst` (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.
 
 <!--
 ### If this is a patch to the `vendor` directory...

README.md

@@ -56,7 +56,7 @@ Or, if you\'re using FreeBSD:
 Or, if you\'re using Gentoo:
 
     sudo emerge pipenv
-    
+
 Or, if you\'re using Void Linux:
 
     sudo xbps-install -S python3-pipenv

news/5261.bugfix.rst

@@ -0,0 +1 @@
+Fix "The Python interpreter can't be found" error when running ``pipenv install --system`` with a python3 but no python.

pipenv/utils/shell.py

@@ -446,6 +446,7 @@ def project_python(project, system=False):
         python = project._which("python")
     else:
         interpreters = [system_which(p) for p in ("python", "python3")]
+        interpreters = [i for i in interpreters if i]  # filter out not found interpreters
         python = interpreters[0] if interpreters else None
     if not python:
         click.secho("The Python interpreter can't be found.", fg="red", err=True)