Remove unused import. (#6996)

`_virtualenv.py` doesn't need to import `__future__.annotations`, as it
has none.

Removing the import:

* Restores the action of the VIRTUALENV_PATCH on Python 3.6

* Eliminates 24 lines of error messages displayed by Python 3.6 when it
starts in an environment created by uv:

```plaintext
Error processing line 1 of /tmp/tmp.ENwqZ0oeyb/lib/python3.6/site-packages/_virtualenv.pth:

  Traceback (most recent call last):
    File "~/.pyenv/versions/3.6.15/lib/python3.6/site.py", line 168, in addpackage
      exec(line)
    File "<string>", line 1, in <module>
    File "/tmp/tmp.ENwqZ0oeyb/lib/python3.6/site-packages/_virtualenv.py", line 3
      from __future__ import annotations
                                       ^
  SyntaxError: future feature annotations is not defined

Remainder of file ignored
```

(Python displays the errors above twice.)

I appreciate the Python team no longer support Python 3.6, but
RedHat-style Linux distributions will support Python 3.6 in their
`/usr/libexec/platform-python` until [releasever 8 expires in
2029](https://access.redhat.com/support/policy/updates/errata#RHEL8_Planning_Guide).
I'm happy for the community to move on, in general, but don't see the
harm in helping those who can't.

I'm not yet sure what in the “remainder of file ignored” is necessary
for my project's build, as I haven't yet finished digging that from
under Hatch. I'll follow up on #6426 when I do, so we can concentrate on
getting to the happy cow.

## Test Plan

```sh
( set -eu
  export VIRTUAL_ENV="$(mktemp -d)"
  ./target/release/uv venv "$VIRTUAL_ENV" --python=python3.6
  ./target/release/uv pip install cowsay        
  $VIRTUAL_ENV/bin/python -m cowsay --text 'Look, a talking cow!' )
  ```
  
Happy output:

```plaintext
Using Python 3.6.15 interpreter at: ~/.local/bin/python3.6
Creating virtualenv at: /tmp/tmp.VHl4XNi3oI
Activate with: source /tmp//tmp.VHl4XNi3oI/bin/activate
Resolved 1 package in 929ms
Installed 1 package in 17ms
 + cowsay==6.0
  ____________________
| Look, a talking cow! |
  ====================
                    \
                     \
                       ^__^
                       (oo)\_______
                       (__)\       )\/\
                           ||----w |
                           ||     ||
```

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

crates/uv-virtualenv/src/_virtualenv.py

@@ -1,7 +1,5 @@
 """Patches that are applied at runtime to the virtual environment."""
 
-from __future__ import annotations
-
 import os
 import sys
 

crates/uv/tests/pip_install.rs

@@ -212,7 +212,7 @@ dependencies = ["flask==1.0.x"]
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       File "[CACHE_DIR]/builds-v0/[TMP]/core.py", line 159, in setup
         dist.parse_config_files()
-      File "[CACHE_DIR]/builds-v0/[TMP]/_virtualenv.py", line 22, in parse_config_files
+      File "[CACHE_DIR]/builds-v0/[TMP]/_virtualenv.py", line 20, in parse_config_files
         result = old_parse_config_files(self, *args, **kwargs)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       File "[CACHE_DIR]/builds-v0/[TMP]/dist.py", line 631, in parse_config_files