Relax Python floor to 3.7

Lowers python_requires and setup.py guard from 3.8 to 3.7, adds the 3.7
Trove classifier, and splits the Cython build requirement so 3.7 users
pin to Cython 3.0.x (3.1+ dropped 3.7). Also tightens the
requirements.txt filter in setup.py to '://' so environment markers on
dependency lines don't accidentally leak into extras_require['source'].

No source changes were needed: a scan of src/lxml turned up no
3.8-only syntax or typing imports outside of already-guarded fallbacks.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: icanhasmath

INSTALL.txt

@@ -41,7 +41,8 @@ see below.
 Requirements
 ------------
 
-You need Python 3.8+ for lxml 6.0 and later.
+You need Python 3.7+ for this Python 3.7-compatible build of lxml 6.0.
+Upstream lxml 6.0 and later require Python 3.8+.
 You need Python 3.6+ for lxml 5.0 and later.
 lxml versions before 5.0 support Python 2.7 and 3.6+.
 

pyproject.toml

@@ -1,5 +1,9 @@
 [build-system]
-requires = ["Cython>=3.2.4", "setuptools"]
+requires = [
+    "Cython>=3.2.4; python_version>='3.8'",
+    "Cython>=3.0,<3.1; python_version<'3.8'",
+    "setuptools",
+]
 
 [tool.cibuildwheel]
 build-verbosity = 1

requirements.txt

@@ -1 +1,2 @@
-Cython>=3.2.4
+Cython>=3.2.4; python_version>='3.8'
+Cython>=3.0,<3.1; python_version<'3.8'

setup.py

@@ -7,8 +7,8 @@
 # for command line options and supported environment variables, please
 # see the end of 'setupinfo.py'
 
-if sys.version_info[:2] < (3, 8):
-    print("This lxml version requires Python 3.8 or later.")
+if sys.version_info[:2] < (3, 7):
+    print("This lxml version requires Python 3.7 or later.")
     sys.exit(1)
 
 from setuptools import setup
@@ -50,10 +50,10 @@ def static_env_list(name, separator=None):
     branch_link = ""
 
 with open("requirements.txt", "r") as f:
-    deps = [line.strip() for line in f if ':' in line]
+    deps = [line.strip() for line in f if '://' in line]
 
 extra_options = {
-    'python_requires': '>=3.8',  # NOTE: keep in sync with Trove classifier list below.
+    'python_requires': '>=3.7',  # NOTE: keep in sync with Trove classifier list below.
 
     'extras_require': {
         'source': deps,
@@ -233,6 +233,7 @@ def build_packages(files):
         'Programming Language :: Cython',
         # NOTE: keep in sync with 'python_requires' list above.
         'Programming Language :: Python :: 3',
+        'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: 3.10',