gh-109653: Improve import time of importlib.metadata / email.utils (p…

…ython/cpython#114664)

My criterion for delayed imports is that they're only worth it if the
majority of users of the module would benefit from it, otherwise you're
just moving latency around unpredictably.

mktime_tz is not used anywhere in the standard library and grep.app
indicates it's not got much use in the ecosystem either.

Distribution.files is not nearly as widely used as other
importlib.metadata APIs, so we defer the csv import.

Before:
```
λ hyperfine -w 8 './python -c "import importlib.metadata"'
Benchmark 1: ./python -c "import importlib.metadata"
  Time (mean ± σ):      65.1 ms ±   0.5 ms    [User: 55.3 ms, System: 9.8 ms]
  Range (min … max):    64.4 ms …  66.4 ms    44 runs
```

After:
```
λ hyperfine -w 8 './python -c "import importlib.metadata"'
Benchmark 1: ./python -c "import importlib.metadata"
  Time (mean ± σ):      62.0 ms ±   0.3 ms    [User: 52.5 ms, System: 9.6 ms]
  Range (min … max):    61.3 ms …  62.8 ms    46 runs
```

for about a 3ms saving with warm disk cache, maybe 7-11ms with cold disk
cache.

importlib_metadata/__init__.py

@@ -3,7 +3,6 @@
 import os
 import re
 import abc
-import csv
 import sys
 import json
 import zipp
@@ -522,6 +521,10 @@ def make_file(name, hash=None, size_str=None):
 
         @pass_none
         def make_files(lines):
+            # Delay csv import, since Distribution.files is not as widely used
+            # as other parts of importlib.metadata
+            import csv
+
             return starmap(make_file, csv.reader(lines))
 
         @pass_none

newsfragments/+.feature.rst

@@ -0,0 +1 @@
+Improve import time (python/cpython#114664).