We retry several kinds of network request failures, but it's often
unclear whether a request was retried or not
(astral-sh#3514 (comment)).
This PR adds a small intermediary layer that logs all transient request
failures, adding the `DEBUG Transient request failure` lines:

```
DEBUG Searching for Python interpreter in virtual environments
DEBUG Found CPython 3.12.3 at `/home/konsti/projects/uv/.venv/bin/python3` (active virtual environment)
DEBUG Using Python 3.12.3 environment at .venv/bin/python3
DEBUG Acquired lock for `.venv`
DEBUG At least one requirement is not satisfied: tqdm
DEBUG Using registry request timeout of 30s
DEBUG Solving with target Python version 3.12.3
DEBUG Adding direct dependency: tqdm*
DEBUG No cache entry for: https://pypi.org/simple/tqdm/
DEBUG Transient request failure for https://pypi.org/simple/tqdm/, retrying: Request error: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
DEBUG Transient request failure for https://pypi.org/simple/tqdm/, retrying: Request error: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
DEBUG Transient request failure for https://pypi.org/simple/tqdm/, retrying: Request error: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
DEBUG Transient request failure for https://pypi.org/simple/tqdm/, retrying: Request error: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
error: Could not connect, are you offline?
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
```

I decided for multi-line logging to show the complete error trace since
only `Transient request failure for https://pypi.org/simple/tqdm/,
retrying: Request error: error sending request for url
(https://pypi.org/simple/tqdm/)` doesn't tell you the actual problem (a
dns error).

Note that running with `-v` will not show messages about retry backoff
timing, but running with `RUST_LOG=debug` now shows a complete picture:

```
DEBUG starting new connection: https://pypi.org/
DEBUG resolving host="pypi.org"
DEBUG Transient request failure for https://pypi.org/simple/tqdm/, retrying: Request error: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
WARN Retry attempt #2. Sleeping 528.728192ms before the next attempt
```

Fixes astral-sh#3572